firefox-gnome-theme/theme/extensions/tab-center-reborn.css

@supports -moz-bool-pref("gnomeTheme.extensions.tabCenterReborn") {
    :root {
        --delay: 0s;
        --transition-time: 0.2s;
        --positionX1: 48px;
        --positionX2: absolute;
    }

	#tabbrowser-tabs {
		display: none !important;
	}

    #browser {
        position: relative;
    }

    #sidebar-box:not([lwt-sidebar]){
        appearance: unset !important;
    }

    #sidebar-box[sidebarcommand*="tabcenter"] {
        z-index: 1;
    }

    #sidebar-box[sidebarcommand*="tabcenter"] #sidebar-header {
        visibility: collapse;
        display: none;
    }

    [sidebarcommand*="tabcenter"] #sidebar, #sidebar-box[sidebarcommand*="tabcenter"]  {
        min-width: 48px !important;
        max-width: 48px !important;
    }

    #sidebar-box[sidebarcommand*="tabcenter"]:not([hidden]) {
        display: block;
        position: var(--positionX2);
        box-sizing: content-box;
        min-width: 48px;
        max-width: 48px;
        overflow: hidden;
        
        border-right: 1px solid var(--sidebar-border-color);
        z-index: 1;
        top: 0;
        bottom: 0;
    }

    #main-window[inFullscreen] #sidebar-box[sidebarcommand*="tabcenter"]:not([hidden]) {
        min-width: 1px;
        max-width: 1px;
    }

    #sidebar-box[sidebarcommand*="tabcenter"]:hover #sidebar,
    #sidebar-box[sidebarcommand*="tabcenter"]:hover {
        min-width: 10vw !important;
        width: 30vw !important;
        max-width: 200px !important;
        z-index: 1 !important;
        transition: all var(--transition-time) ease var(--delay);
    }

    #sidebar-box[sidebarcommand*="tabcenter"]:not(:hover) #sidebar,
    #sidebar-box[sidebarcommand*="tabcenter"]:not(:hover) {
        transition: all var(--transition-time) ease 0s;
    }

    @media (width >= 1200px) {
        #sidebar-box[sidebarcommand*="tabcenter"]:hover #sidebar,
        #sidebar-box[sidebarcommand*="tabcenter"]:hover {
            max-width: 250px !important;
        }
    }

    [sidebarcommand*="tabcenter"] ~ #sidebar-splitter {
        display: none;
    }

    [sidebarcommand*="tabcenter"] #sidebar {
        max-height: 100%;
        height: 100%;
    }

    #main-window:not([inFullscreen]) #sidebar-box[sidebarcommand*="tabcenter"]:not([hidden]) ~ #appcontent {
        margin-left: var(--positionX1);
    }

    #main-window[inFullscreen][inDOMFullscreen] #appcontent {
        margin-left: 0;
    }

    #main-window[inFullscreen] #sidebar {
        height: 100vh;
    }

    [sidebarcommand*="tabcenter"] #sidebar-header {
        background: #0C0C0D;
        border-bottom: none !important;
    }

    [sidebarcommand*="tabcenter"] ~ #sidebar-splitter {
        border-right-color: #0C0C0D !important;
        border-left-color: #0C0C0D !important;
    }

    [sidebarcommand*="tabcenter"] #sidebar-switcher-target,
    [sidebarcommand*="tabcenter"] #sidebar-close {
        filter: invert(100%);
    }

    @media (max-width: 630px) {
        #urlbar-container {
            min-width: 100% !important;
        }
    
        #menubar-items {
            display: none !important;
        }
    }
    
    #main-window[inFullscreen="true"] #sidebar-box[sidebarcommand="tabcenter-reborn_ariasuni-sidebar-action"],
    #main-window[inFullscreen="true"] #sidebar-box[sidebarcommand="tabcenter-reborn_ariasuni-sidebar-action"] + splitter {
        visibility: collapse;
        animation: var(--transition-time) fadeOut;
        animation-fill-mode: forwards;
    }

    @keyframes fadeOut {
        0% {
            visibility: visible;
            opacity: 1;
        }
        
        100% {
            opacity: 0;
        }
    }
}

@supports -moz-bool-pref("gnomeTheme.extensions.tabCenterReborn.alwaysOpen") {
    #sidebar-box[sidebarcommand*="tabcenter"] #sidebar,
    #sidebar-box[sidebarcommand*="tabcenter"] {
        min-width: 10vw !important;
        width: 30vw !important;
        max-width: 250px !important;
        z-index: 1 !important;
        transition: all var(--transition-time) ease var(--delay);
    }
    
    #sidebar-box[sidebarcommand*="tabcenter"]:not([hidden]) {
        position: relative;
        margin-right: -50px;
    }
    
    @media (width >= 1200px) {
        #sidebar-box[sidebarcommand*="tabcenter"]:hover #sidebar,
        #sidebar-box[sidebarcommand*="tabcenter"]:hover {
            max-width: 250px !important;
        }
    }
}