Depends. How the forum displays is a function of both your wp theme and your sp theme. 

We get basic layout info from the wp theme. It determines the page layout since the forum appears on a wp page. So width for example is set by the wp theme. 

Then the forum theme takes over. It still inherits the wp theme css but then defines its own special styles for appearance.  So the basic color and font stuff will be the same. 

If you previously matched the forum colors (i.e. overlay) to the wp theme you may want to adjust to match the new wp theme.  

The overall forum structure will remain unchanged – assuming it has the width to do what it did before