diff --git a/webextensions/sidebar/styles/square/plain-color-dark.css b/webextensions/sidebar/styles/square/plain-color-dark.css
index eeb8439e6..2a9a33505 100644
--- a/webextensions/sidebar/styles/square/plain-color-dark.css
+++ b/webextensions/sidebar/styles/square/plain-color-dark.css
@@ -49,13 +49,13 @@
   --tab-highlighter-inactive: var(--grey-10-a20);
 }
 
-  /* Firefox applies dark colors to scrollbars in its in-content UI, when a
-     Dark theme is applied by the platform. To imitate such appearance we need
-     to specify dark colors manually. See also:
-     https://github.com/piroor/treestyletab/issues/2373 */
-  #tabbar {
-    scrollbar-color: var(--in-content-button-background-active-mixed) var(--in-content-box-background);
-  }
+/* Firefox applies dark colors to scrollbars in its in-content UI, when a
+   Dark theme is applied by the platform. To imitate such appearance we need
+   to specify dark colors manually. See also:
+   https://github.com/piroor/treestyletab/issues/2373 */
+#tabbar {
+  scrollbar-color: var(--in-content-button-background-active-mixed) var(--in-content-box-background);
+}
 
 :root[color-scheme="system-color"] #background::after {
   content: none;