Also fix the section title style in theme_aware.md to be consistent.

Original Author: tluk@

Bug: none
Change-Id: I54b113efb27ec2e98026765c2b11c0bb6e3d2385
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2297736
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789284}