Fix bad border colour on computed styles
Fix bad border colour on computed styles

Because the border declaration was missing a defined colour it was not
inverted correctly in dark mode, meaning the border would be black which
is not easily spotted when in dark mode. To fix this I picked out the colour that was being used in light mode and explicitly defined it.

By declaring a specific black colour, it is then inverted into white in
dark mode and it's much clearer as the screenshots below show (no light mode included as this change is a no-op in light mode).

Before, dark mode (note the barely visible black border around the
margin): https://imgur.com/OuaagvW

After, dark mode: https://imgur.com/ecntqHp

Fixed: 1106683
Change-Id: I2a834d23f964efaa6757636460b8b5136bfdfb23
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2308548
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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