Not to discourage you, but you pretty much need to understand CSS if you want to customize the Bootstrap framework. Bootstrap studio does allow you to change the colors on certain elements that you place, but with pre-designed templates, many of the colored elements are set in the Bootstrap CSS file and are "locked", meaning they cannot be changed through the visual editing controls. In some cases, the !important modifier is also used.
You have to copy the specific CSS rules you want to change to a new CSS file which will then override the Bootstrap CSS rules via the "cascade."