[WebUI] Managed UI: dark mode support

This adds dark mode support to <managed-footnote> by using standard
colors.

Bug: 919102
Change-Id: I797a46086b72df78720a6e1ecae18c9e0debb7fe
Reviewed-on: https://chromium-review.googlesource.com/c/1427573
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625061}
1 file changed