WebUI Dark Mode: style <select class="md-select"> for all pages

Screenshots: https://imgur.com/a/zioTkR5
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g3b4ff0456d_0_9

R=dpapad@chromium.org
BUG=883049

Change-Id: I1e38c095af75ffbf574046891e5ad0427b4b4855
Reviewed-on: https://chromium-review.googlesource.com/c/1427456
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625041}
1 file changed