fixup! BB 32308: Use direct browser sizing for letterboxing.

TB 44289: Handle the case where --toolbar-field-color is set to
"inherit".

Also, better handle "currentColor" values to not rely on the computed
color, which may vary with the UI state.

Also, replace "/*" comments with "//".


(cherry picked from commit 961b3271e9af455b91bb43e8ea18701ff6a5577f)

Co-authored-by: Henry Wilkes <henry@torproject.org>