How much custom CSS are we able to add into Point?

Point can be customized quite a lot, however, it might conflict with future updates. Less custom styles means that you run a lower risk of something not working as it should with future releases/updates. 

UI builder is a supporting application which works as a "programing" tool to change a significant number of CSS values. This tool is commonly updated during onboarding and on request if your brand changes. If you have questions about your portal styling, system color palettes, fonts etc. contact your CSM or support and they will assist you further.