[a11y] Show a visile focus outline when stepping through the styles tab

The styles tab supports keyboard navigation over interactive controls.
Currently this is invisible so this CL is adding a focus outline.

Bug: 396311936
Change-Id: I5169639f1f8977fd7a3f0db2b9dd3cc17de4bdb9
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6286010
Reviewed-by: Changhao Han <[email protected]>
Commit-Queue: Philip Pfaffe <[email protected]>

Author: pfaffe

front_end/panels/elements/stylePropertiesTreeOutline.css

@@ -296,3 +296,7 @@ devtools-icon.open-in-animations-panel {
     color: var(--icon-css-hover);
   }
 }
+
+.value :focus {
+  outline: 1px solid var(--sys-color-outline);
+}