widget: Set style background on realize

vte analogue to https://bugzilla.gnome.org/show_bug.cgi?id=692988

(cherry picked from commit 16bda596da75356686991da2ee75e5f9f501cdac)
ring: Fix compiler warnings

(cherry picked from commit f275e61df7deeb93bf47c99ce24a46a2ffdd559d)
Version 0.36.1