Add inverted scrollbar styles

src/definitions/modules/dimmer.less

@@ -69,6 +69,23 @@
   border-radius: inherit !important;
 }
 
+/* Scrollbars */
+.addScrollbars() when (@useCustomScrollbars) {
+  .ui.dimmer:not(.inverted)::-webkit-scrollbar-track {
+    background: @trackInvertedBackground;
+  }
+  .ui.dimmer:not(.inverted)::-webkit-scrollbar-thumb {
+    background: @thumbInvertedBackground;
+  }
+  .ui.dimmer:not(.inverted)::-webkit-scrollbar-thumb:window-inactive {
+    background: @thumbInvertedInactiveBackground;
+  }
+  .ui.dimmer:not(.inverted)::-webkit-scrollbar-thumb:hover {
+    background: @thumbInvertedHoverBackground;
+  }
+}
+.addScrollbars();
+
 /*******************************
             States
 *******************************/

src/themes/default/globals/site.variables

@@ -157,6 +157,12 @@
 @thumbInactiveBackground: rgba(0, 0, 0, 0.15);
 @thumbHoverBackground: rgba(128, 135, 139, 0.8);
 
+/* Inverted */
+@trackInvertedBackground: rgba(255, 255, 255, 0.1);
+@thumbInvertedBackground: rgba(255, 255, 255, 0.25);
+@thumbInvertedInactiveBackground: rgba(255, 255, 255, 0.15);
+@thumbInvertedHoverBackground: rgba(255, 255, 255, 0.35);
+
 /*-------------------
   Highlighted Text
 --------------------*/