GTK3:
Improve support for headerbars styling (Allan Day, Paolo Borelli,
Jakub Steiner)
Increase legibility of pushed button style (Allan Day)
Fix CSD shadows for dark themed windows (Matthias Clasen)
Fix dim-label color on selected rows (Thomas Wood)
Fix font style for context menus (Matthias Clasen)
Fixes for destructive/suggested-action buttons (Andreas Nilsson)
App specific fixes for Baobab, Nautilus (Stefano Facchini, António Fernandes)
HighContrast:
Add support for CSD and headerbars
Fix insensitive menu items styling
Fix extra frame for GtkStatusbar
App specific fixes for Nautilus
