Dialer: restore full compatibility with theme chooser

Change-Id: Id6d3d7d97d5c136779711eca66cc29cf59a2e205
Dialer: improve letters' readability

Change "sans-serif-light" to "sans-serif" and make color a bit darker.

Change-Id: I14449f99cdff8efbdcfc1910fe91fe5539f971fe