I googled the problem & tried the solution here: https://wordpress.org/support/topic/...e-whole-screen
Now my site technically filled the page, but the content was crammed in the middle. So not an actual solution. See this pic:
It also made the smileys HUGE (filling the whole width of the post). So I undid this change again…
Other guides I found went over my head :/
It does resize when I make the browser window smaller, so I guess the maximum width is the problem, but… Well, as you can tell: I have no experience with CSS and don’t know how to fix it.