You probably do have a functions.php file within your theme's code editor. The only thing that scares me is how easy it can be to mess up your entire website with one line of code out of place. I would consider installing a plug-in to try code out such as Simple Custom CSS, that plug-in overrides any code within your theme. If you don't know much about CSS I would push you away from editing your actual theme's code, unless you know how to do a child theme to edit and try stuff out. Just go for the plug-in and see if it fixes your problem.
I'd say to that it could be a plug-in itself messing things up. Try disabling them all and see if things start working again. Then re-activate on-by-one to see if the problem persists.