I would recommend you decide on a template for joomla, and then decide on a theme you like for SMF, preferably with images that are similar to your joomla colors.  In the Joomla CSS you can always change the width of the page to allow more room for the forums in wrapped mode.  Many people who integrate SMF into Joomla will combine the CSS files together and remove any duplicate entries, then just edit the SMF entries to mirror the site.  Afterwards pick apart the actual theme code and remove parts you may not want such as the heading or login boxes. It takes some work doing things this way, but the site comes out clean and definently less chaotic.