Child theme should be used by more advanced users who knows how it works. Basically it is for those who modify theme files (php, html) and don't want to lose those changes while future updates. All details about child theme you can read on https://codex.wordpress.org/Child_Themes
But css's can be overwritten under Theme options > Custom CSS & JS > Custom CSS section what is much better in our opinion because all settings are stored in database.
Comments
there could be 2 reasons:
1) You are using pre-defined skin instead of Custom. Please notice that custom colors works only with custom skin.
2) You are using cache stuff and that's why you do not see any changes in front-end.