Defining new heading styles beyond H2 and H3

Thanks to the excellent guide on changing the visual style of h5p, I've been able to adapt the look of my material to how I like. In the future I plan on making quite a lot of h5p content. So far I've changed the css for heading 2 and heading 3 but eventually I'm going to want to use different styles (without altering the style of stuff I've already made). Is there any way that I can either define new heading types (h4, h5 etc) or point some of my content to a different css file?

Thanks for any pointers anyone is able to give me.