Remove sass extend compounds

[hrdwrrsk]

• this will allow this theme to compile with sassc 3.6.0 (although some distros are still using old version of sassc, for exampe: Arch Linux still uses sassc version 3.5.x (3.6.0 in testing?), and Ubuntu 18.04 LTS)

[SaucySauce]

I'm on Manjaro stable branches which are slower than Arch and I'm already on 3.6.1-1.