As far as I know, there is no such option in the backend.
I’d recommend to get used to CSS since it’s very useful for designing a shop.
Just google “how to include CSS in Shopware” in order to get the directory and stuff right.
If that’s the only thing you want to change in your shop, you can just copy&paste the code but if u want it to be responsive you have to check the different options for example for smartphones etc.
You should read a quick CSS guide for Shopware, it wont usually take longer than 1 hour to understand what’s going on there.