This post details my thoughts on where to start with branding for SharePoint 2013. In particular, I think Themes (.spcolor, .spfont) are now usable, and recommend to start there, linking to a couple of resources. But first, I talk about what not to do.
What not to do
It might seem like a good idea to get a graphic design company to develop a whiz-bang look for your intranet, then turn around and ask a web development company to turn it into a SharePoint branding. Often this starts out with an HTML reset or HTML boilerplate.
The core SharePoint stylesheets have over 10,000 lines of CSS -- unless you want to rewrite all of that, you do not want to be doing a CSS reset. You never know when BI dashboard widget XYZ is going to need to display a green/red traffic light based on some CSS buried deep within the core files.
Don't do it.