Make panel support dark mode

Slightly adjust Panel related theme/styles to support dark mode.

Screenshot:
Wifi Panel: https://screenshot.googleplex.com/5FJ1wwMjgrn.png
NFC Panel: https://screenshot.googleplex.com/AhNJhEMXCS7.png
Volume Panel: https://screenshot.googleplex.com/iyOxAz9NJPv.png
Connectivity Panel: https://screenshot.googleplex.com/pvExHZVNW7m.png

Test: Visual/Manual verification
Fixes: 130815366
Change-Id: Ic96898091c52ccebf4437d96099ad553884d1eb4