Minify css and js file option can be found in Theme Options – Advanced – Site Speed.

IMPORTANT

*Each time you make changes with CSS, you will need to disable Minify css and js file option, make the change that you wish and after enable the option again.

**If you do not follow the above steps, the change that you did with css will not be considered.

*** Child theme style.css will not be loaded if you use this setting on YES as child theme modifications unless you modify the child theme. See this help 

How to make child theme changes to work with theme Minifiy option enabled

Site speed