Site skins do not get updated when upgrading mojoPortal. This is by design as we don't want the installer over-writing any changes that one may make to their skin(s).
If you haven't modified the skins at all, you could copy the contents of the \data\skins directory to \data\sites\[site-id]\skins.
If you have modified the skins, you should refer to the "Important Skin Changes" forum post which details changes to skins in recent releases.
To address your specific problem with the forum. I think you need to add the following the style.config of your skin:
<file cssvpath="/Data/style/forums/style.css" imagebasevpath="/Data/style/forums/">none</file>