Assuming you’ve not already modified the core CSS file, in Publii, just go to: Tools & Plugins > Custom CSS and enter the following overrides:
.cookie-popup__save {background: #000}
.cookie-popup--uses-badge {background: #000}
Note: the above example changes the GDPR button(s) background to black (#000) (for the non-AMP site version). However, remember, these buttons should stand out and contrast from the rest of the website design. So black may not be ideal.