Bug 1879815 - Consistently use non-native theme in-content. r=desktop-theme-reviewers,jules

Differential Revision: https://phabricator.services.mozilla.com/D201612

layout/style/res/scrollbars.css

@@ -21,6 +21,7 @@
     -moz-control-character-visibility: initial;
     -moz-min-font-size-ratio: initial;
     -moz-box-collapse: initial;
+    -moz-theme: initial;
 
     /* We don't want zoom on our ancestors to affect our styles. */
     zoom: document;

toolkit/themes/shared/in-content/common-shared.css

@@ -104,6 +104,7 @@
   --card-outline-color: var(--grey-30);
   --card-shadow-hover: var(--card-shadow), 0 0 0 5px var(--card-outline-color);
 
+  -moz-theme: non-native;
   accent-color: var(--in-content-accent-color);
   color-scheme: light dark;
 }