--- a/devtools/client/debugger/new/dist/debugger.css
+++ b/devtools/client/debugger/new/dist/debugger.css
@@ -242,20 +242,16 @@ body {
   border-radius: 8px;
   background: transparent;
 }
 
 ::-webkit-scrollbar-thumb {
   border-radius: 8px;
   background: rgba(113, 113, 113, 0.5);
 }
-
-:root.theme-dark .CodeMirror-scrollbar-filler {
-  background: transparent;
-}
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
 
 menupopup {
   position: fixed;
   z-index: 10000;
   background: white;

--- a/devtools/client/sourceeditor/codemirror/mozilla.css
+++ b/devtools/client/sourceeditor/codemirror/mozilla.css
@@ -286,8 +286,13 @@ selector in floating-scrollbar-light.css
 
 .CodeMirror-Tern-farg-current {
   text-decoration: underline;
 }
 
 .CodeMirror-Tern-fhint-guess {
   opacity: .7;
 }
+
+/* Override the background-color: white applied to filler elements in codemirror.css */
+.CodeMirror-scrollbar-filler, .CodeMirror-gutter-filler {
+  -moz-appearance: scrollcorner;
+}