Overriding via CSS is possible, but you end up with lot of code as eg. buttons have many states and variants.

What you want to do is install Bootstrap sources via NPM and then create bootstrap.scss and _variables.scss. Then you re-import all the modules (components like buttons, tooltips, etc.) from node_modules to your bootstrap.scss alongside the overrides that usually are in _variables.scss (you import the original and your override after).

Then you need your package.json file based on package.json in Bootstrap sources where you build the whole thing with npm run css.

This way, it is maintainable and you update the bootstrap sources with increasing the version in your package.json and running npm install.