tree-style-tabs.css

/*
https://medium.com/@Aenon/firefox-hide-native-tabs-and-titlebar-f0b00bdbb88b
 */

/* hides the native tabs */
#TabsToolbar {
    visibility: collapse;
}

/* To hide horizontal bars */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar {
    opacity: 0;
    pointer-events: none;
}

#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;
}

/* For Tree Style Tab */
#sidebar-header{
    font-size: 1em !important;
    padding: 5px 2px 5px 13px !important;
}

/*
https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#auto-showhide-sidebar-by-mouseover-hover
*/
/* Change styling of pending (unloaded) tabs */
tab-item.discarded tab-item-substance {
    opacity: 0.75;
}

tab-item.discarded .label-content {
    color: red;
}

tab-item {
    --tab-size: 25px !important;
}
tab-item  tab-item-substance {
    height: var(--tab-size);
}

/*Collapse in default state and add transition*/
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] {
    overflow: hidden;
    min-width: 40px !important;
    max-width: 40px !important;
    transition: all 0.2s ease;
    border-right: 1px solid #0c0c0d;
    z-index: 2;
}

/*Expand to 260px on hover*/
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]:hover,
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar {
    min-width: 260px !important;
    max-width: 260px !important;
    z-index: 1;
}

/* show separator space between root level tabs */
tab-item[data-level="0"] {
    margin-top: calc(var(--tab-size) / 3); /* please change this as you like */
}
/* cancel needless separators on exceptions */
tab-item[data-level="0"].pinned,
tab-item[data-level="0"].pinned + tab-item[data-level="0"],
tab-item[data-level="0"] + tab-item[data-level="0"]:not([data-child-ids]) {
    margin-top: 0;
}