widget: Set style background on realize
widget: Set style background on realize

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

(cherry picked from commit 16bda596da75356686991da2ee75e5f9f501cdac)

pjm0616/vte,pjm0616/vte,pjm0616/vte