Thank You @webbersky. Being a noob I was troubled how to make a child theme (https://codex.www.ads-software.com/Child_Themes), so I altered the files directly. But at least I understood how to input these changes and now I have a one column theme at full width.
Only thing thats a bit off is that I get an extra ‘–>’ at the left bottom of my site, as you can see at https://oivaopisto.org/
