Revert "don't use deprecated gtk_style_context_get_background_color"

This reverts commit 33ad43fdc4edeabea1c9f710f746680458cf425c.
don't use deprecated gtk_style_context_get_background_color

Replace gtk_style_context_get_background_color with local static
function get_background_color that does exactly same thing.

Thanks to Lucian Langa for reporting bug.