You would need to create appropriate HTML markup in the layout.master, write up CSS files for each of mojoPortal's features (blog, forums, etc.) that you are going to use on the site, and you might need to configure the theme.skin file such that mojo will generate the correct markup to match whatever css you used for various things (pagers, menus, etc.), which might require some study for somebody who isn't familiar with it.
If you're looking for help, i7MEDIA can convert a current site to a new, fully hand-crafted and custom built skin. We can also convert PSD's or HTML Templates to mojoPortal skins.
Hope this helps,