helm/users/profiles/firefox/treestyletab.css

:root{ 
  --dark-0: #1A1E28;
  --dark-base: #1F2430;
  --dark-1: #2A3141;
  --dark-2: #323A4E;
  --dark-3: #424D67;
  --dark-4: #4A5673;
  --light-0: #E8E7E3;
  --light-base: #CCCAC2;
  --light-1: #C3C0B7;
  --light-2: #B9B7AC;
  --light-3: #B0ADA0;
  --light-4: #A7A395;
  --accent: #FFCC66;
  --yellow: #695380;
  --green: #D5FF80;
  --red: #f28779;
  --extension-icon-mask: grayscale(85%) invert(30%) sepia(13%) saturate(1019%) hue-rotate(184deg) brightness(95%) contrast(96%);
}

:root.incognito{ 
  --dark-0: #1C0E34;
  --dark-base: #20103c;
  --dark-1: #2F1D4E;
  --dark-2: #38225D;
  --dark-3: #4A2D7B;
  --dark-4: #53338A;
  --light-0: #CC6B9C;
  --light-base: #B9407C;
  --light-1: #AB3B73;
  --light-2: #9C3569;
  --light-3: #8D305E;
  --light-4: #7D2B54;
  --accent: #e9207e;
  --yellow: #C9B336;
  --green: #36C987;
  --red: #C9365D;
  --extension-icon-mask: grayscale(85%) invert(17%) sepia(33%) saturate(3632%) hue-rotate(245deg) brightness(92%) contrast(90%);
}


:root{
    --tab-border-radius: 7px; /* border radius of tabs */
    --animation-duration: 200ms; /* duration of different animations [0s: turn all animations off] */
    --spacing: 14px; /* spacing between tabs. [<15px: compact tabs] */
    --distance-from-edge: 10px; /* distance between tabs, and left-right edges of sidebar*/
    --hover-text-spacing: 0.1; /* should be left alone. with hover sidebar, if text is visible in collapsed status, increase this */

    --ease-in: cubic-bezier(0.32, 0, 0.67, 0);
    --ease-out: cubic-bezier(0.22, 1, 0.36, 1);
    --ease-in-out: cubic-bezier(0.65, 0, 0.35, 1);

    --collapsed-width: 60px;
}

/************UNCUSTOMIZED CSS************/
#tabbar-container #tabbar{
   margin-bottom: 15px !important;
}

:root, #background{
    background:  var(--dark-base) !important;
}

#all-tabs{
   margin: 10px var(--distance-from-edge);
}

tab-item:not(.collapsed) {
    margin-top: var(--spacing);
    border-radius: var(--tab-border-radius);
    border: none !important;
    padding-top: 9px;
    padding-bottom: 10px;
}

tab-item.collapsed{
    height: 0;
    margin: 0 !important;
    padding: 0 !important;
}

tab-item:not([data-level="0"]):not(.pinned){
    border-left: dashed 2px var(--light-4) !important;
    border-top-left-radius: 0;
    border-bottom-left-radius: 0;
    margin-top: calc(var(--spacing)/2);
}

tab-item tab-favicon{
    left: 0;
    filter: var(--extension-icon-mask);
    transition: left calc(var(--animation-duration)*2) var(--ease-out);
}

.highlighter {
    display: none !important;
}

tab-item .label {
    color: var(--light-2) !important;
    padding-bottom: 2px;
}


tab-item .twisty:before{
   background: var(--light-3) !important;
}

tab-item.active .label {
    color: var(--light-0) !important;
}

tab-item.active {
    background: var(--dark-3) !important;
}


tab-item:not(.active).highlighted{
    background: var(--dark-1) !important;
}

tab-item:not(active):hover {
    background: var(--dark-1);
}

tab-item:hover tab-closebox {
    right: 10px;
    opacity: 1;
}

tab-item:not(pinned) tab-closebox {
    position: absolute;
    margin-top: 2px;
    height: 20px;
    width: 20px;
    right: -30px;
    border-radius: 50%;
    padding-top: 2px;
    padding-left: 2px;
    background: var(--light-4);
    transition   : all var(--animation-duration) var(--ease-out);
}

.sound-button:after {
    background: var(--light-3) !important;
    margin-top: 2px;
}

.counter{
    color: var(--light-4) !important;
    background: var(--dark-2) !important;
    border-radius: 7px;
    margin: 2px 2px 0px 0px;
    padding: 0 4px;
    padding-bottom: 2px;
    order: -2 !important;
}

.counter:before, .counter:after{ content: ""}

/********LEFT CONTENT TAB(static)*************/

:root.left tab-item{
    padding-left: 0;
}

:root.left tab-item tab-favicon {    
    transform: scale(120%);
    margin-right: 5px;
    left: 80%;
}

:root.left tab-item .label {
    transform: translateX(10px);
    right: 28%;
}


:root.left tab-item[data-child-ids] .twisty{
    margin-right: 5px;
}

/********RIGHT CONTENT TAB(hover)*************/

:root.right tab-item .label {
    margin-left: 10px;
    margin-right: calc(var(--collapsed-width)*0.3) !important;
}

:root.right tab-item:not(pinned):hover tab-closebox{
    right: calc(var(--collapsed-width)*0.3);
}

:root.right tab-item:not([data-level="0"]):not(.pinned){
    border: none !important;
    border-right: dashed 2px var(--light-4) !important;
    border-radius: var(--tab-border-radius);
    border-top-right-radius: 0;
    border-bottom-right-radius: 0;
}

:root.right tab-item.active:not([data-level="0"]):not(.pinned){
   margin-right: 5px !important;
}

:root.right .sound-button{
    order: -1 !important;
}

:root.right tab-item.active.pinned {
    position: relative;
    left: 190px !important;
}

:root.right tab-item.active.pinned tab-favicon {
    transform: scale(130%) !important;
    margin-left: 7px;
}

/***********************************/

.tab
    .favicon
    .favicon-default::before {
    filter: var(--extension-icon-mask); // change for light theme
}

.tab[data-current-favicon-uri="undefined"]
    .favicon
    .favicon-default::before{
    background: url("chrome://branding/content/identity-icons-brand.svg") no-repeat center !important;
    mask: none !important;
}

/***********NEW TAB BUTTON**********/

.newtab-button-box {
    border: none !important;
    margin: 0 50px;
}

.newtab-button {
    border: none !important;
    border-radius: 8px;
    padding: 8px 0 !important;
    margin: 0 5px;
}

.newtab-button::before{
   background: var(--light-3) !important;
}

.newtab-action-selector-anchor {
    border: none !important;
    margin-right: 10px;
    margin-top: 3px;
}

.newtab-button:hover {
    background: var(--dark-1);
}

/**********PINNED TAB***************/

tab-item.pinned {
    margin: 5px !important;
    padding: 5px !important;
    transition   : all 0.1s var(--ease-out);
}

/***********************************/

#all-tabs {
    border: none !important;
}