GitLab

Use var for dark mode dialog background color and drop shadow.

Change-Id: I31a4821966456d13b1d433b5999d182e1781922e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692331
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675877}