Yaru: Provide a mixed theme

[Feichtmeier]

Some thoughts:

Provide a mixed theme, based on the light theme but with inverted headerbar and headerbar button colors.

This needs probably a change to our meson build, with a new file "_headerbar.scss" which only gets imported when the mixed theme is build.

The gtk2 theme could be copied from the light theme

[chrisjbillington]

For what it's worth, a convention in some other themes is to call such a mixed theme "darker". So the mixed theme might be called "Yaru-darker"

See for example: https://github.com/arc-design/arc-theme

[clobrano]

The post fix -dark might be used in the future for a better support for dark theme and the darker one would be the dark variant, so it'd be a problem. However I believe that yaru-ambiance could be indicative enough ;)

[clobrano]

closed by #1532