Themes no longer buildable with libsass 3.6.2+

Frederik noticed that the CI fails for win32 due to syntax changes (as nobody touched HC in millenia). Will need to investigate.

[109/1291] Linking target gdk/libgdk-3-0.dll.
[110/1291] Generating symbol file gdk/ffe01f7@@gdk-3@sha/libgdk-3-0.dll.symbols.
[111/1291] Generating HighContrast-inverse with a custom command.
FAILED: gtk/theme/HighContrast/gtk-contained-inverse.css 
"C:/msys64/mingw32/bin/sassc.EXE" "-a" "-M" "-t" "compact" "../gtk/theme/HighContrast/gtk-contained-inverse.scss" "gtk/theme/HighContrast/gtk-contained-inverse.css"
compound selectors may no longer be extended.
Consider `@extend ${compound.components.join(', ')}` instead.
See http://bit.ly/ExtendCompound for details.
compound selectors may no longer be extended.
ERROR: Job failed: execution took longer than 1h0m0s seconds