WebUI Dark Mode, Extensions: fix load error UI colors

<extension-code-section> is used from 2 places in extensions (AFAICT),
when showing where runtime errors come from and when showing errors
trying to load an extension (i.e. manifest errors).

This CL should confine "light mode" to just the actual code parts, and
show the load errors in dark mode.

Screenshots: https://imgur.com/a/gF1gd5D

R=dpapad@chromium.org
BUG=936524

Change-Id: I99b9cc56df1712c04023206a493a883d42d6fb8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497804
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@{#637588}
3 files changed