WebUI Dark Mode: rework link and error colors on Extensions page

Pretty much everything should use --cr-link-color, which magically
changes based on light/dark mode.

There was also just a ton of duplicated a[href] rules, which I
instead added to shared_style.html (and verified each place I
deleted from used include="shared-style").

Screenshots: https://imgur.com/a/kaT6MJc


Change-Id: I00440e1bcfaae21b31763f51d65385729ba7fab9
Reviewed-on: https://chromium-review.googlesource.com/c/1429379
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625386}
8 files changed