trilium/src/public/stylesheets/tree.css

ul.fancytree-container {
    padding-left: 0;
}

.fancytree-title {
    margin-left: 7px !important;
}

.fancytree-expander {
    visibility: hidden;
}

.fancytree-folder .fancytree-expander {
    visibility: visible;
}

.fancytree-node {
    padding: 4px;
    border: 1px solid transparent;
    display: block;
    cursor: pointer;
    overflow: hidden;
    white-space: nowrap;
    text-overflow: ellipsis;
}

.fancytree-node:not(.fancytree-loading) .fancytree-expander {
    background: none;
    height: auto;
    text-align: center;
}

.fancytree-node:not(.fancytree-loading) .fancytree-expander:before {
    font-family: 'boxicons' !important;
    speak: none;
    font-size: x-large;
    text-transform: none;
    line-height: 1;
    content: "\e9b2";
    position: relative;
    top: 2px;
    margin-right: 5px;
}

/* this is done to preserve correct indentation. Better solution would be preferable */
.fancytree-node:not(.fancytree-folder) .fancytree-expander:before {
    color: var(--main-background-color); /* setting to background color makes this invisible */
}

.fancytree-node.fancytree-expanded .fancytree-expander:before {
    font-family: 'boxicons' !important;
    content: "\e9ac";
}

/** some common text styling for cssClass label */
span.fancytree-node.underline .fancytree-title { text-decoration: underline; }
span.fancytree-node.dotted .fancytree-title { text-decoration: dotted; }
span.fancytree-node.bold .fancytree-title { font-weight: bold; }
span.fancytree-node.muted { opacity: 0.6; }

/** following will hide ancestors of hoisted (filtered) note */
.fancytree-submatch:not(.fancytree-match) {
    display: none !important;
}

/** resets indent of hoisted note */
.fancytree-submatch:not(.fancytree-match) + ul {
    padding: 0 !important;
}

ul.fancytree-container {
    outline: none !important;
    background-color: inherit !important;
}

.fancytree-custom-icon {
    font-size: 1.2em;
}

span.fancytree-title {
    color: inherit !important;
    background: inherit !important;
    outline: none !important;
}

span.fancytree-node.protected > span.fancytree-custom-icon {
    filter: drop-shadow(2px 2px 2px var(--main-text-color));
}

span.fancytree-node.multiple-parents .fancytree-title::after {
    content: " *"
}

span.fancytree-node.fancytree-active-clone:not(.fancytree-active) .fancytree-title {
    font-weight: bold;
}

/* first nesting level has lower left padding to avoid extra left padding. Other levels are not affected */
.ui-fancytree > li > ul {
    padding-left: 5px;
}

.ui-fancytree ul {
    padding-left: 20px;
}

span.fancytree-active .fancytree-title {
    font-weight: bold;
    border: 0;
}

span.fancytree-active {
    border-color: var(--main-border-color) !important;
    border-radius: 5px;
}

span.fancytree-active, span.fancytree-active.fancytree-selected {
    color: var(--active-item-text-color) !important;
    background-color: var(--active-item-background-color) !important;
    border-color: var(--main-background-color) !important; /* invisible border */
    border-radius: 5px;
}

span.fancytree-selected {
    color: var(--hover-item-text-color) !important;
    background-color: var(--hover-item-background-color) !important;
    border-color: var(--main-background-color) !important; /* invisible border */
    border-radius: 5px;
    font-style: italic;
}

span.fancytree-node:hover {
    border-radius: 5px;
    border: 1px solid var(--main-border-color);
}

.fancytree-title:hover, span.fancytree-node:hover .fancytree-title {
    border: 0;
}

span.fancytree-node.archived {
    opacity: 0.6;
}

.fancytree-node:hover .bx.tree-item-button {
    display: inline-block;
}

.bx.tree-item-button {
    display: none;
    font-size: 120%;
    cursor: pointer;
    position: relative;
    left: 5px;
    top: 2px;
}

.tree-item-button:hover {
    border-color: var(--main-border-color);
}

.add-note-button {
    display: none;
}

span.fancytree-node:hover .add-note-button {
    display: inline-block;
}