GuildPortal has just been dramatically improved when it comes to using custom HTML + CSS. First, we've added a "Navigation" selection in the Control Panel (under Custom HTML). Clicking that takes you to three editors. You can modify the code that is output before navigation items (pages on your site), the code that is output for navigation items, and the code that is output after navigation items.
Once you specify the code, you can then use the Full Page custom HTML editor's new tag, $block[nav=html], and your pages will be rendered as the page loads, using your mark-up.
Also, three more tags have been added to the Full Page HTML: leftwidgets, centerwidgets, and rightwidgets. You no longer have to stick with designs that render the main widget body in a tabular format, and can control exactly where your widgets appear.
A new option has been added that prevents GuildPortal from rendering any style information based on your point-and-click settings in the Style Tools area of the Control Panel. This makes it so the only style info comes from your custom CSS, without any interference from GP, except for a few helper rules for margins and the like. You can enable this option in the Control Panel by expanding Custom HTML, selecting Custom CSS, selecting the Full Custom CSS tab, and checking the box at the bottom that says "Disable all GuildPortal CSS Output".
These enhancements, in combination with the already existing Banner, Footer, Widgets, and Full Page custom HTML and Custom CSS, provide complete control over how your site looks. Now you can bring your favorite templates from anywhere -- and there are a lot of free template sites out there. Oh, hey, and if you find any really great ones and port them to your GuildPortal site, share them, it's easy! Just open the Control Panel, expand Style Tools, and then click Share Theme.
Here's a snap of a theme developed using custom HTML and CSS from a freely-available one meant for a completely different system: