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Following https://crrev.com/c/1748123, we stopped picking up the
preferenceTheme attribute on v26+, causing incorrect styling. This
change fixes that, and slightly reorganizes the Preference themes into
base styles to avoid some repetition.

Bug: 860848
Change-Id: Ib6b3b0b4394f483ef219deb2b8b1d9c63622b258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1774315Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691813}