You will get the css file in paticular moodle theme by
your moodle -> theme -> your theme -> style -> some.css
After any customization on css file of theme, for reflect you need to do purge all cache by
Site administration > Development > Purge all caches
Because moodle does cache for CSS and javaScript to increase the performance.
If you will not comfort with purge all cache after every changes in css, then you can enable the 'Theme Designer Mode ` option on theme setting page. You can get this page by
Site Administration > Appearance > Themes > Theme settings