Migrated from `@extend` to `@include`

Migrated from `@extend` to `@include`

`@extend` has a few downsides that led to this change:
- selectors are merged, thus the order in which individual CSS appears
in the stylesheet is no longer deterministic
- doesn't work with `@media`
- creates really huge selectors that negatively impact the work with
browser developer tools

Using `@include` does increase the raw filesize as the individual CSS
properties are now copied rather than referenced: Gzip negates this
almost entirely.