Happened to me so just a warning that updating the Quark template to latest version deletes your overriride.css file!
thankfully I had an akeeba backup...as it was heavily modded, a lot of months work would of been lost...
Easy solution is to open gk_quark_J!3.zip with winzip or similar and rename 'override.css' to '_override.css'
This saves your existing file from deletion on update.
Hope this saves someone a headache! lol
peace
Dok