Add background color to Force specific element state button when focused

https://imgur.com/a/PicsDQn

Adding background color to force specific element state button so it's
visible when it's focused for accessibility purpose.

Bug: 407812148
Change-Id: Idded70a4b1cffa4a7d481af4ccdc6fadf35ec678
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6423126
Reviewed-by: Simon Zünd <[email protected]>
Reviewed-by: Jack Franklin <[email protected]>
Commit-Queue: Jack Franklin <[email protected]>

Author: Jayson Chen

front_end/panels/elements/elementStatePaneWidget.css

@@ -45,3 +45,7 @@
   grid-template-columns: repeat(2, 1fr);
   margin-bottom: 4px;
 }
+
+.force-specific-element-header:focus {
+  background-color: var(--sys-color-state-hover-on-subtle);
+}