[GTK] Fix dark background on widgets using XWT backend
[GTK] Fix dark background on widgets using XWT backend

Fixes bug #41865 - Grey borders around text entry in
Project Options > Configurations > Start Action

antmicro/xwt,TheBrainTech/xwt,akrisiun/xwt,hamekoz/xwt,cra0zy/xwt,lytico/xwt,hwthomas/xwt,mono/xwt,residuum/xwt