Did you have a look at this part of the documentation: https://docs.mendix.com/howto/front-end/customize-styling-new/? I think that is a good starting point for your custom styling of pages.
As a starting point, you can try to change the following variables in your project folder / them / web / custom-variables.
These colors are used by for example buttons on your pages. After you have changed them they should be automatically injected into your browser.