call-window: use colors from the theme for placeholders
call-window: use colors from the theme for placeholders

Use color shades from the color theme for preview placeholders, instead
of hardcoding black/red.

https://bugzilla.gnome.org/show_bug.cgi?id=672561

GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets