Change selected animation row background color to be transparent
Change selected animation row background color to be transparent

The transparency was lost during the dark mode migration and when a row
is selected the scrubber is chopped as a result. This replaces the
background color of the selected row to a variable which is partially
transparent.

Before: https://imgur.com/a/uc4Xm7e
After: https://imgur.com/a/6l6sWRh

Fixed: 1235961
Change-Id: I18aa4f4e40def5220c7b3caf687041656d51e74a
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3068106
Reviewed-by: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Andres Olivares <9d3bededfc9b98cb289557338a8f98e77d597607@chromium.org>

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