I breath Everforest. Mochi is a delight to use as is, but when I discovered I could edit the theme, I went full on to implement my favorite. It's a work in progress but I did not want to wait any longer to share it. The source code is available on GitHub, but you can follow the instructions below if GitHub is not your thing. In the mean time, I'd appreciate your input.

How to use:

• Save the code below into a .css file.
• Go to Settings > Appearances > Custom CSS them.
• Click Add theme and select the theme file.

Notes:

• I chose the medium variant for dark mode, and the hard variant for light mode. If you want to change this, follow the instructions in the GitHub repository.
• If you edit the theme file, you will have to re-import it for the changes to take effect.
• I tried using Mochi variables (defined with --mochi--*) as much as possible, but sometimes I had to target some elements directly. There are hundreds of these variables and I might have missed what I needed.
• There is much room for improvement. A theme guide from the incredible developer would be much appreciated :)

/* -------------------------------------------- */
/* ---------------- Root styles --------------- */
/* -------------------------------------------- */
html.dark {
  /* Everforest colors */
  --bg-dim: #232a2e;
  --bg0: #2d353b;
  --bg1: #343f44;
  --bg2: #3d484d;
  --bg3: #475258;
  --bg4: #4f585e;
  --bg5: #56635f;
  --bg-visual: #543a48;
  --bg-red: #514045;
  --bg-yellow: #4d4c43;
  --bg-green: #425047;
  --bg-blue: #3a515d;
  --bg-purple: #4a444e;
  --fg: #d3c6aa;
  --red: #e67e80;
  --orange: #e69875;
  --yellow: #dbbc7f;
  --green: #a7c080;
  --aqua: #83c092;
  --blue: #7fbbb3;
  --purple: #d699b6;
  --grey0: #7a8478;
  --grey1: #859289;
  --grey2: #9da9a0;
  --statusline1: #a7c080;
  --statusline2: #d3c6aa;
  --statusline3: #e67e80;
  /* Mochi variables */
  --mochi--card--background: var(--bg1);
  --mochi--card-box--background: var(--bg1);
  --mochi--button--background: var(--bg1);
  --mochi--side-bar--background: var(--bg-dim);
  --mochi--card--background-hover: var(--bg-blue);
  --mochi--box-select--background: var(--bg0);
  --mochi--box-select--background-hover: var(--bg-blue);
  --mochi--side-bar-icon--color: var(--blue);
  --mochi--review-history--row-background-dark: var(--bg-dim);
  --mochi--button-hover--background: var(--bg-blue);
  --mochi----border-color: var(--bg3);
  --mochi--side-bar-item-selected--background: var(--bg1);
}

html:not(.dark) {
  /* Everforest colors */
  --bg-dim: #f2efdf;
  --bg0: #fffbef;
  --bg1: #f8f5e4;
  --bg2: #f2efdf;
  --bg3: #edeada;
  --bg4: #e8e5d5;
  --bg5: #bec5b2;
  --bg-visual: #f0f2d4;
  --bg-red: #ffe7de;
  --bg-yellow: #fef2d5;
  --bg-green: #f3f5d9;
  --bg-blue: #ecf5ed;
  --bg-purple: #fceced;
  --fg: #5c6a72;
  --red: #f85552;
  --orange: #f57d26;
  --yellow: #dfa000;
  --green: #8da101;
  --aqua: #35a77c;
  --blue: #3a94c5;
  --purple: #df69ba;
  --grey0: #a6b0a0;
  --grey1: #939f91;
  --grey2: #829181;
  --statusline1: #93b259;
  --statusline2: #708089;
  --statusline3: #e66868;
  /* Mochi variables */
  --mochi--card--background: var(--bg0);
  --mochi--card-box--background: var(--bg0);
  --mochi--button--background: var(--bg0);
  --mochi--side-bar--background: var(--bg3);
  --mochi--card--background-hover: var(--bg-green);
  --mochi--box-select--background: var(--bg2);
  --mochi--box-select--background-hover: var(--bg5);
  --mochi--side-bar-icon--color: var(--grey2);
  --mochi--review-history--row-background-dark: var(--bg3);
  --mochi--button-hover--background: var(--bg5);
  --mochi--side-bar-item-selected--background: var(--bg5);
}

html body {
  --fonts-monospace: UbuntuMono Nerd Font Mono;
  --mochi--review-button--background: var(--mochi--side-bar--background);
  --mochi--card--color: var(--fg);
  --mochi--md-code--background: var(--bg2);
  --mochi--cloze-2--background-color: var(--mochi--side-bar--background);
  --mochi----background-color: var(--bg0);
  --mochi--jump-to--selection-bg: var(--bg2);
  --mochi--show-more--background: var(--mochi--side-bar--background);
  --mochi--show-more--background-hover: var(--bg5);
  --mochi--progressbar--foreground: var(--bg5);
  --mochi--inverted-button--background: var(--grey2);
  --mochi--inverted-button--count-background: var(--grey0);
  --mochi--inverted-button-hover--background: var(--grey1);
  --mochi--progressbar--foreground-red: var(--statusline3);
  --mochi--re-review--icon: var(--red);
  --mochi--card--border-color-active: var(--bg5);
}

/* -------------------------------------------- */
/* ------------- memo_views edits ------------- */
/* -------------------------------------------- */
/* Main content background */
.memo_views_frame_frame-container {
  background-color: var(--bg1);
  background-size: 50px 50px;
  /* Background pattern - dots */
  background-image: radial-gradient(
    circle,
    hsl(37, 38%, 83%) 1px,
    rgba(0, 0, 0, 0) 1px
  );
}
.dark .memo_views_frame_frame-container {
  background-color: var(--bg0);
  background-image: radial-gradient(
    circle,
    hsl(31, 27%, 25%) 1px,
    rgba(0, 0, 0, 0) 1px
  );
}

/* Fix dashbaord overview background anomoly */
.memo_views_pages_layout_frame-content > div:nth-child(3) {
  background-color: initial !important;
}

/* Show More gradient */
.memo_views_atoms_card_tile-fade {
  background: linear-gradient(0deg, var(--bg0), var(--bg0) 33px, transparent);
}
.dark .memo_views_atoms_card_tile-fade {
  background: linear-gradient(0deg, var(--bg1), var(--bg1) 33px, transparent);
}
.memo_views_card_list-view_card-container:hover
  .memo_views_atoms_card_tile-fade,
.memo_views_card_grid-view_card-container:hover
  .memo_views_atoms_card_tile-fade {
  background: linear-gradient(
    0deg,
    var(--bg-green),
    var(--bg-green) 33px,
    transparent
  );
}
.dark
  .memo_views_card_list-view_card-container:hover
  .memo_views_atoms_card_tile-fade,
.dark
  .memo_views_card_grid-view_card-container:hover
  .memo_views_atoms_card_tile-fade {
  background: linear-gradient(
    0deg,
    var(--bg-blue),
    var(--bg-blue) 33px,
    transparent
  );
}

/* Editor background */
.memo_views_modal_card_editor-background {
  background-color: var(--bg1);
  font-size: 0.95em !important;
}
.dark .memo_views_modal_card_editor-background {
  background-color: var(--bg0);
}

/* Cards font color */
.memo_views_modal_modal-dialog {
  color: var(--fg);
}

/* Fix filter menu colors in dark mode */
.dark .memo_views_atoms_menu_menu {
  color: rgba(255, 255, 255, 0.7);
}

/* Cloze backgrounds */
.memo_views_diagrams_diagram-cloze {
  background-color: var(--grey1);
}

/* Make foot note jumps smooth */
.memo_views_modal_modal-container {
  scroll-behavior: smooth;
}

/* 3D button effect */
.memo_views_atoms_button_default-button,
.memo_views_atoms_button_inverted-button {
  box-shadow:
    inset 0px 0px 0px 1px rgba(0, 0, 0, 0.05),
    inset 0px -3px 0px 0px rgba(0, 0, 0, 0.15) !important;
}
.dark .memo_views_atoms_button_default-button,
.dark .memo_views_atoms_button_inverted-button {
  box-shadow:
    inset 0px 0px 0px 1px rgba(0, 0, 0, 0.05),
    inset 0px -3px 0px 0px rgba(0, 0, 0, 0.25) !important;
}

/* Grid view */
.memo_views_card_grid-view_container {
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
}

/* Column view */
.memo_views_card_column-view_left-column-container {
  width: 330px;
  border-left: none;
  border-right: none;
}

.memo_views_pages_deck_breadcrumbs-container {
  border: none !important;
}

.memo_views_card_column-view_right-column-container {
  color: var(--fg);
  background-color: var(--mochi--card--background);
  padding: 0; /* Remove the think border in editing mode */
}

.memo_views_card_column-view_column-row-markdown-container h1,
.memo_views_card_grid-view_card-container h1 {
  font-size: 1.1em !important;
  font-weight: 600 !important;
}
.memo_views_card_column-view_column-row-markdown-container h2,
.memo_views_card_column-view_column-row-markdown-container h3,
.memo_views_card_column-view_column-row-markdown-container h4,
.memo_views_card_grid-view_card-container h2,
.memo_views_card_grid-view_card-container h3,
.memo_views_card_grid-view_card-container h4 {
  font-size: 1em !important;
}

/* Reset opacity of cards in column view */
.memo_views_card_column-view_column-row-markdown-container .paragraph {
  opacity: 1;
}

/* Remove divider lines in note footer in column view */
.memo_views_molecules_card-info_content-wrapper > div + div {
  border: none;
  margin-top: 0;
}

/* Remove box shadow */
.memo_views_card_column-view_column-row-container {
  box-shadow: none;
}

.memo_views_card_column-view_right-column-outer-container {
  border-radius: 8px;
  margin: 10px 10px 10px 0;
  border: 1px solid var(--mochi----border-color);
}

.memo_views_card_column-view_editor-styles {
  padding: 3em 3em 30vh 3em; /* Compensate for removing outer padding */
}

.memo_views_card_column-view_side-class {
  padding: 3em 3em 5em; /* Match padding to editing mode */
}

/* -------------------------------------------- */
/* ----------- memo_markdown edits ------------ */
/* -------------------------------------------- */
/* Cards font family */
/* Tags - inline and in card status */
.memo_markdown_components_tag-link {
  font-family: var(--fonts-monospace);
  padding: 3px;
  border: 1px solid var(--bg4);
}

/* Inline code style */
code:not(pre code):not(kbd code) {
  color: var(--blue) !important;
  border: 1px solid;
  white-space: nowrap; /* prevent work break for inline code */
  margin-right: 0.1em; /* prevent trailing punctuations touching the border */
}

/* Highlighted text with == */
.memo_markdown_styles_markdown-class mark {
  border-radius: 0;
  color: var(--bg0);
  background-color: var(--aqua);
  background-image: none;
  padding: 0.2em;
  margin: initial;
  vertical-align: middle;
}

/* Reference card links */
.memo_markdown_components_inline-link {
  /* Custome styles */
  background: var(--bg2);
  border: 1px solid var(--bg4);
}
.dark .memo_markdown_components_inline-link {
  background: var(--bg3);
}
.dark .memo_markdown_components_inline-link:hover {
  background: var(--bg4);
}

/* Increase monospace font */
/* Only for UbutntuMono Nerd Mono font because it's inherently smaller */
.memo_markdown_styles_markdown-class code,
.memo_markdown_styles_markdown-class pre {
  font-size: 0.95em !important;
}

/* Used for footnote legends */
.memo_markdown_components_blue-link {
  opacity: 0.9;
}
h1 .memo_markdown_components_blue-link,
h2 .memo_markdown_components_blue-link,
h3 .memo_markdown_components_blue-link,
h4 .memo_markdown_components_blue-link,
h5 .memo_markdown_components_blue-link,
h6 .memo_markdown_components_blue-link {
  font-size: initial;
}

/* -------------------------------------------- */
/* ----------- Raw HTML tags edits ------------ */
/* -------------------------------------------- */
strong {
  color: var(--statusline3);
}

em {
  color: var(--aqua);
}

strong em {
  color: var(--purple);
}

/* ----------------- Headings ----------------- */
/* General */
h1,
h2,
h3,
h4,
h5,
h6 {
  font-family: "Avenir Next Condensed";
  font-weight: 500 !important;
  text-wrap: pretty; /* Not yet supported in the Electron version Mochi is using */
}

/* Font size */
h2 {
  font-size: 1.8em !important;
}

h3 {
  font-size: 1.5em !important;
}

h4 {
  font-size: 1.3em !important;
}

h5 {
  font-size: 1em !important;
}

h6 {
  font-size: 1em !important;
}

/* Rainbow headings */
h1 {
  color: var(--fg);
}

h2 {
  color: var(--red);
}

h3 {
  color: var(--aqua);
}

h4 {
  color: var(--orange);
}

h5 {
  color: var(--blue);
}

h6 {
  color: var(--yellow);
}

/* -------- Custom keyboard input style ------- */
kbd {
  font-family: var(--fonts-monospace);
  color: var(--statusline3) !important;
  background: var(--bg2);
  border-radius: 4px;
  padding: 0.2em 0.45em;
  margin-right: 0.1em;
  white-space: nowrap;
}
.dark kbd {
  background: var(--bg0);
}
kbd {
  box-shadow:
    inset 0px 0px 0px 1px rgba(0, 0, 0, 0.05),
    inset 0px -3px 0px 0px rgba(0, 0, 0, 0.15) !important;
}
.dark kbd {
  box-shadow:
    inset 0px 0px 0px 1px rgba(0, 0, 0, 0.05),
    inset 0px -3px 0px 0px rgba(0, 0, 0, 0.25) !important;
}

/* ------------------ Tables ------------------ */
table {
  border-radius: 5px;
  overflow: hidden;
}

thead tr {
  background-color: var(--bg-green);
}

tbody tr:nth-child(odd) {
  background-color: var(--bg2);
}

tbody tr:nth-child(even) {
  background-color: var(--bg1);
}

/* ------------ Custom list styles ------------ */
ol > li,
ul > li {
  position: relative;
}

ol > li {
  list-style-type: none;
}

/* Custom numbered bullets */
ol > li::before {
  --bullet-size: 1.3rem;
  content: counter(list-item);
  position: absolute;
  top: 0.4rem;
  inset-inline-start: -1.7rem;
  width: var(--bullet-size);
  height: var(--bullet-size);
  line-height: var(--bullet-size);
  font-size: 0.75rem;
  font-weight: 600;
  text-align: center;
  color: var(--fg);
  background-color: var(--bg2);
  border-radius: 99rem;
  box-shadow: inset 0 0 0 1px var(--bg4);
  font-family: Cascadia Code;
}
.dark ol > li::before {
  background-color: var(--bg3);
  border-radius: 99rem;
  box-shadow: inset 0 0 0 1px var(--bg5);
}

/* Indent lines */
ol > li::after,
ul > li::after {
  content: "";
  position: absolute;
  top: 2rem;
  bottom: 5px;
  inset-inline-start: -1.1rem;
  width: 1px;
  background-color: var(--bg4);
}

ul > li::after {
  inset-inline-start: -0.9rem;
}

/* Experimental */
hlr,
hla {
  font-family: var(--fonts-monospace);
  padding: 2px 6px;
  border-radius: 3px;
  border: 1px solid;
  background-color: var(--bg2);
}

hlr {
  color: var(--red);
}

hla {
  color: var(--orange);
}

/* ------------------------------------------- */
/* ----------- Syntax highlighting ------------*/
/* ------------------------------------------- */
.hljs {
  color: var(--fg) !important;
  background: var(--bg-dim) !important;
  padding: 0.3em !important;
}

.hljs-comment,
.dark .hljs-comment,
.hljs-quote,
.dark .hljs-quote {
  color: var(--grey1);
}

.hljs-doctag,
.dark .hljs-doctag,
.hljs-keyword,
.dark .hljs-keyword,
.hljs-formula,
.dark .hljs-formula,
.hljs-number,
.dark .hljs-number {
  color: var(--purple);
}

.hljs-section,
.dark .hljs-section,
.hljs-name,
.dark .hljs-name,
.hljs-selector-tag,
.dark .hljs-selector-tag,
.hljs-deletion,
.dark .hljs-deletion,
.hljs-subst,
.dark .hljs-subst {
  color: var(--red);
}

.hljs-literal,
.dark .hljs-literal {
  color: var(--aqua);
}

.hljs-string,
.dark .hljs-string,
.hljs-regexp,
.dark .hljs-regexp,
.hljs-addition,
.dark .hljs-addition,
.hljs-attribute,
.dark .hljs-attribute,
.hljs-meta-string,
.dark .hljs-meta-string {
  color: var(--green);
}

.hljs-built_in,
.dark .hljs-built_in,
.hljs-class .hljs-title,
.dark .hljs-class .hljs-title {
  color: var(--yellow);
}

.hljs-attr,
.dark .hljs-attr,
.hljs-variable,
.dark .hljs-variable,
.hljs-template-variable,
.dark .hljs-template-variable,
.hljs-type,
.dark .hljs-type,
.hljs-selector-class,
.dark .hljs-selector-class,
.hljs-selector-attr,
.dark .hljs-selector-attr,
.hljs-selector-pseudo,
.dark .hljs-selector-pseudo {
  color: var(--orange);
}

.hljs-symbol,
.dark .hljs-symbol,
.hljs-bullet,
.dark .hljs-bullet,
.hljs-link,
.dark .hljs-link,
.hljs-meta,
.dark .hljs-meta,
.hljs-selector-id,
.dark .hljs-selector-id,
.hljs-title,
.dark .hljs-title {
  color: var(--blue);
}

Yea like Garrett said, I found them in the developer tools, one at a time :)

Hi Sayed, thanks for the theme, it is gorgeous!

Can I ask, how did you reverse engineer the --mochi-- variables and classes? I couldn't find any documentation on them.

Not OP, but you can see the --mochi-- variables and classes (and all the other HTML/CSS/JS) in the Developer Tools, just go to View > Toggle Developer Tools.

Thanks both, much appreciated :)