Custom CSS should reset when empty.
Custom CSS should reset when empty.

Fixes #2876.

smblott-github/vimium,poacher2k/vimium,gdh1995/vimium,decaffeinate-examples/vimium,gdh1995/vimium,philc/vimium,smblott-github/vimium,poacher2k/vimium,poacher2k/vimium,philc/vimium,decaffeinate-examples/vimium,gdh1995/vimium,philc/vimium,decaffeinate-examples/vimium,smblott-github/vimium