Basically it just making a theme for SMF.
The added sideblocks and article template comes "afterwards" and draws upon the SMF theme.
But they don't have to. I plan to make some examples of layouts/tempaltes that can either be a normal "portal" look, like BlocZone - or look like a site which seem to have the forum as just a section, not revealing that it IS in fact the engine.
Its all in the templates.
The one thing that is missing for that to be possible, is section-specific blocks. So that a block can be on a single page only, and that page actually seem like a stand-alone page with some info drawn into it.
I guess more like an CMS than a portal.
....but back to your question about tutorials on making themes..I will try to do some , but as IchBin says, it does take some work to write up.