Where should I search uncompressed .css?
when I go to sites/default/files/css/ I find only set of gzipped .css files


You need to change your configuration.

Go to Configuration > Performance

and unchecked the aggregate and compress CSS files box, Save Configuration.



You then need to go to /sites/all/themes/YOURTHEMENAME/css folder and hopefully you should see .css files now. (Assuming you are using Drupal 7 and NOT using Drupal 8)

(might need to re-start FileZilla to seem them, assuming you are using FileZilla that is.)