I tried the tip from @karlmistelberger in the New forum - big change for the worse topic and after installing Stylus it was a piece of cake to get the width completely used:
- Navigate to a page on this forum
- Press the newly added S button
- Hover with your mouse to forums.opensuse.org in the newly popped-up window and click on it
- In the Add Style window that will pop up, paste the code shared by @karlmistelberger
- Done
Still I think it would be a good idea to change the defaults used for this forum so everyone benefits.