Yes that is possible, the problem is that Joomla! and SMF are using the same classes. When loading your forum in wrapped mode both Css files are loaded so the are conflicting at this moment. 

just search for the Class that correspond with text and modify them. You can use for searching for the correct classes the Firefox Webdeveloper toolbar. (a great tool for web developers)

You will find a lot off info about this on the forum