Add style and width to `.expandable-view-title` elements' top border
Add style and width to `.expandable-view-title` elements' top border

The value of the border-top was migrated to a themeColors.css variable,
however it was not visible because it was lacking style and width values.

Bug: chromium:1152736
Change-Id: I6cd9e4b9255b310fe1f2ec14be11092263f6d4e7
Fixed: 1206248
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2876583
Commit-Queue: Andres Olivares <9d3bededfc9b98cb289557338a8f98e77d597607@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Auto-Submit: Andres Olivares <9d3bededfc9b98cb289557338a8f98e77d597607@chromium.org>
Reviewed-by: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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