You didn’t say what else had changed and clearly something has because you are also getting two copies of the forum being displayed one under the other. This is most usually caused by a theme doing something non-standard.
So the first thing is to deal with that and that might actually correct the issue anyway.
Go to the forum admin > Integration > Page and Permalink panel. Down near the bottom you will find the section entitled ‘Theme Display Options’. Turn on one of the top two and run the forum to see if that fixes the double display. If it doesn’t try the second option instead. You shouldn’t need both but might do.
That will hopefully fix the double display and might well fix the font size issue as well.