[WifiSetup] Fix colors during setup

Fixed background color for dark theme, Wi-Fi icon color for light
and dark themes, "Add another network" button color for light and
dark themes. Created wifi_signal_colored which uses wifi_signal_color
to tint the Wi-Fi icon.
Also fixed theme extra propagation.

Bug: 16903606
Change-Id: Iab24828f888d7827184762d0d63d139d0a2c381e