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