dark_mode: Add myself as the owner.

Bug: None
Change-Id: Ia980d35c49adc85e7bbca908c6e6a6438d668a57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468572
Commit-Queue: Min Chen <minch@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816792}

ash/style/OWNERS

+minch@chromium.org

ash/system/dark_mode/OWNERS

 amehfooz@chromium.org
-tengs@chromium.org
\ No newline at end of file
+minch@chromium.org
+tengs@chromium.org