Fix insufficient contrast for the strings "Dimmer" and "Brighter" in the light theme

Apply same the sub text style from seekbar preference

Bug: 219752953
Bug: 229722937
Bug: 245093240
Fix: 245093240
Test: Manual testing
Change-Id: If107452b942e23dee17c73873f4aedc3a25c7f5d
(cherry picked from commit a8b9c805)