New file

chrome/autohide_sidebar.css

@@ -0,0 +1,29 @@
+/* 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;
+}
+
+#sidebar-splitter{ display: none }
+
+#sidebar-header{ overflow: hidden; }
+
+#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; }
+
+/* Add sidebar divider */
+
+#sidebar,
+#sidebar-header{
+  border-right: 1px solid rgb(80,80,80);
+}

chrome/blank_page_background.css

@@ -0,0 +1,6 @@
+/* Set blank page background-color */
+/* Uses color from theme_colors if available */
+
+#tabbrowser-tabpanels{
+  background-color: var(--uc-light-bkgnd-color,rgb(46,54,69)) !important;
+}

chrome/hide_toolbox_top_bottom_borders.css

@@ -0,0 +1,10 @@
+#main-window[sizemode="normal"]{ border-top: none !important }
+#navigator-toolbox::after{ content: none !important }
+
+/* OPTIONAL - uncomment to remove still remaining space atop of tabs */
+/* This just increases the height of tabs by few pixels, not decrease the toolbar height */
+
+/* Fx pre-65 */
+/* #navigator-toolbox > #TabsToolbar{margin-top: -2px;} */
+/* Fx65+ */
+/* #TabsToolbar > .toolbar-items > spacer{ display: none; } */