I think that this is a "shortfall" of the templates. BUT... it may be done on purpose ! - See, if they allow editing in those, it may be off the screen of a visitor. Some website builders (mostly HTML driven builders like Dreamweaver) have facilities to write code that makes your website Auto Stretch according to the screen it is viewed on.
There is a clever trick you can try .... look at the Design options and then at the Page Background --- you can do the same as i did below - just make sure you know where to put your text as it may dissappear on the background - OR ------- give the Content Image a different solid colour ?