mobile-config-firefox/chrome/autohide_sidebar.css

/* Show sidebar only when the cursor is over it  */
/* The border controlling sidebar width will be removed so you'll need to modify these values to change width */

#sidebar-box{
  --uc-sidebar-width: 40px;
  --uc-sidebar-hover-width: 210px;
  position: relative;
  min-width: var(--uc-sidebar-width) !important;
  width: var(--uc-sidebar-width) !important;
  max-width: var(--uc-sidebar-width) !important;
}

#main-window[sizemode="fullscreen"] #sidebar-box{ --uc-sidebar-width: 1px; }

#sidebar-splitter{ display: none }

#sidebar-header{ overflow: hidden; color: var(--chrome-color, inherit) !important}

#sidebar{
  transition: min-width 115ms linear !important;
  min-width: var(--uc-sidebar-width) !important;
  will-change: min-width;
}

#sidebar-box:hover > #sidebar{ min-width: var(--uc-sidebar-hover-width) !important; }

.sidebar-panel{
  background-color: transparent !important;
  color: var(--newtab-text-primary-color) !important;
}

.sidebar-panel #search-box{
  -moz-appearance: none !important;
  background-color: rgba(249,249,250,0.1) !important; 
  color: inherit !important;
}

/* Add sidebar divider and give it background */

#sidebar,
#sidebar-header{
  background-color: var(--toolbar-bgcolor) !important;
  border-right: 1px solid rgb(80,80,80);
}