[Seamless] Adjust background color for wifi dialog in SUW flow.

This fix that makes the background color of the dialog will be consistent during the SUW flow.

Dark mode
  - WifiDialog: https://hsv.googleplex.com/6050138159054848
  - Set up Offline: https://hsv.googleplex.com/5449263784919040

Light mode
  - WifiDialog: https://hsv.googleplex.com/5350517514436608
  - Set up Offline: https://hsv.googleplex.com/5245039291662336

Bug:298326674
Test: manual
Change-Id: Ifb507a5f3ba8ae3360552117b086e864a1145468