The use of border-width-set here was an attempt to differentiate
between explicitly set (from code / ui files) border width from
theme changes. But when we are calling gtk_window_set_border_width
to apply the theme value, the -set property gets set, and all
further theme changes are ignored. This has the effect of only
letting the default value of these properties get applied.

Fix this by unsetting border-width-set after applying theme values.