So far it's in your hands. You may also check this topic
At this moment, I don't see lots of sense to make a version which could suit 400px and let's say 1800px at once. In the practice, such designs do not work good for mobile devices anyway, but they almost always look ugly on all screens. As far as I know, most of productive websites still have 2 very different layout versions for desktops and mobiles. That always works best. So for the forums, we have the mobile version
, and I've checked it on the plain mobile device many times, it works much better than the "sensitive" version.
If you have other suggestions regarding it, you're welcome.
I'm not about the "mobile-friendly page" here, which could fit the Google guides. "Mobile-friendly" template could be achieved using just a few CSS techniques, and possibly in the future releases, I will do this for miniBB, still not sure how long time it would take. I need to change the primary layout of miniBB for this (again), and honestly speaking, I am not ready for it. It's too much job. I have more interesting stuff around than this one.
So regarding your forum, you could try to work out both the responsive and/or mobile design on your own so far. It highly depends on the primary webpage you have for your forum, also your forum layout, which needs to differ a lot from the default version of miniBB for this case.