Fix findbar background on sites with a translucent theme color

[josiah-keller]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

• Go to a site with an rgba theme color (such as https://stackoverflow.com)
• Press Ctrl-F
• Note that "Match case" label and close button blend seamlessly into the rest of the findbar

Fixes #5742, #5744