Is there a way to change CSS styling of a web page dynamically based on certain values?

Working with Angular 2.

I had an idea today to give users the ability to well select their style. I would setup my app and then have a, I guess service, set certain values and have my css change on selected values. I just can't think of a way to do this. I got the idea from seeing bootswatch and how you can select a theme.

I want to be able to set up a theme and have users be able to preview and select which theme they like. I am making a portfolio page for myself but I want to make it open source and public to see and maybe for people who don't have any programming skills to make a portfolio page on my web app. Except with a lot more editing they can do themselves.