Bug 1871024 - Update in-content border color to use border-color token. r=dao,desktop-theme-reviewers

This addresses the lack of visual separation in about:logins on Ubuntu
in High Contrast mode. The about:logins sidebar having the same
background color as the main body of the page is expected in High
Contrast mode, so we re-establish the borders on the sidebar to provide
visual separation between the elements.

Differential Revision: https://phabricator.services.mozilla.com/D197728

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

@@ -190,7 +190,7 @@
      * In theory, we shouldn't be conveying invalid state just with a colour
      * change... */
     --in-content-border-invalid: ThreeDShadow;
-    --in-content-border-color: ThreeDShadow;
+    --in-content-border-color: var(--border-color);
 
     --in-content-button-text-color: ButtonText;
     --in-content-button-text-color-hover: SelectedItemText;