Time to finally ask here, because I'm sure the answer is obvious...
Brand new theme with base as the parent
Been designing my theme using Firefox and I noticed this only because I wanted to log in on another browser as a student to check how something looked.
How my course topic boxes look in Firefox:
How they look in Chrome and Safari:
As you can see, in Chrome and Safari the right hand side of the topic box is rather... large.
Things I have tried that have fixed it in Chrome/Safari have broken it in Firefox.