Great extension for vertical tabs with high customization.

My sidebery style:

#root.root {--tabs-indent: 16px;}

/* No empty space after last tabs */
.TabsPanel .bottom-space {
    height: 0
}

/* The spaces in the navigation bar take a whole line. It forces a line return */
.NavigationBar .nav-item[data-class="space"][data-type="dynamic"] {
//  height: 8px;
    height: 0;
    width: 100% !important;
}

/* The delimiters are horizontal now and force a line return */
.NavigationBar.-top .nav-item[data-class="space"][data-type="static"] {
    height: 8px;
    width: 100% !important;
}
.NavigationBar.-top .nav-item[data-class="space"][data-type="static"]::after {
    height: 1px;
    width: 100%;
    top: auto;
    left: auto;
    background-color: #75747A;
}

/* Moving Sidebery navigation bar to bottom, to mimic how Space works on Arc Browser. Make sure to choose Horizontal when activating the navigation bar. */
.top-horizontal-box {
  display: flex;
  order: 10; 
}
#search_bar {
    order: 5;
}

/* A bottom border on the pinned tabs bar */
.PinnedTabsBar {
     border-bottom: 1px solid #75747A;
}

.Tab .title {
    font-size: 12px;
    font-family: unset;
}

/* OLD STYLES
#root.root {--tabs-indent: 16px;}

/* No empty space after last tabs */
.TabsPanel .bottom-space {
    height: 0
}

/* The spaces in the navigation bar take a whole line. It forces a line return */
.NavigationBar .nav-item[data-class="space"][data-type="dynamic"] {
//  height: 8px;
    height: 0;
    width: 100% !important;
}

/* The delimiters are horizontal now and force a line return */
.NavigationBar.-top .nav-item[data-class="space"][data-type="static"] {
    height: 8px;
    width: 100% !important;
}
.NavigationBar.-top .nav-item[data-class="space"][data-type="static"]::after {
    height: 1px;
    width: 100%;
    top: auto;
    left: auto;
    background-color: #75747A;
}

/* Moving Sidebery navigation bar to bottom, to mimic how Space works on Arc Browser. Make sure to choose Horizontal when activating the navigation bar. */
.top-horizontal-box {
  display: flex;
  order: 10; 
}
#search_bar {
    order: 5;
}

/* A bottom border on the pinned tabs bar */
.PinnedTabsBar {
     border-bottom: 1px solid #75747A;
}

.Tab .title {
    font-size: 12px;
    font-family: unset;
}
*/