I don't believe these are related issues.

Your forum admin panel will still show subSilver as the default theme because its likely the only theme located in your html/modules/Forums/templates/ folder. The reason your seeing a completely different skin on your forums is because its a .css file located in your html/themes/yourthemename/forums folder which overrides the default subSilver theme.

As for the coding errors in your forums, it sounds like your version of nuke needs to be patched as I had similar issues with a 7.8 install once.

One more question, does a wysiwyg editor come as part of your 7.7 install?