Fix Firefox select dropdowns, maybe

Summary: Fixes T12930. I can't verify this fix, but the colors here work in light/dark mode correctly.

Test Plan: Wait for @cspeckmim to verify

Reviewers: epriestley, cspeckmim

Reviewed By: cspeckmim

Subscribers: cspeckmim, Korvin

Maniphest Tasks: T12930

Differential Revision: https://secure.phabricator.com/D18286