Fix notification styles

No issue

I'm not sure why, but @extend's here using placeholder selectors, not classes, work.

This fixes the notification and modal styles, which meant changing how the default button selector works. It is now a placeholder, and a real class extends that. The modal styles extended the default button style, which can now happen.
Merge pull request #3905 from PaulAdamDavis/fix-extends

Fix notification styles