Hi Marnix,
Certain styles provided within the theme options are meant for generally changing the basics of your site’s appearance.
Once we get into very specific styles, this is where comes in Custom CSS. Otherwise there would be just too many options on the options page and there are endless cool and creative things that can be done with CSS.
I understand CSS may seem a bit daunting at first. But, until you’re comfortable with it or even if you’re never comfortable with it we’ll be here to take requests and write code for you, you only need copy and paste.
Should you want to learn CSS we’ve prepared the following documentation (and appears that you’ve gotten a pretty good handle on it so far):
https://startupwp.com/topic/customizing-your-theme/
Thank You