Creating a theme: CSS only?

The Zen theme probably gives you the most CSS control over your site. The Framework theme is bit simpler but still lets you do a lot. Both start out nice and boring and leave everything up to you. There are a few other barebones-type themes out there, but Zen and Framework are the ones I've tried.

cheers

A contributed theme, unless it is specifically one of the ones made as a framework / starting point theme, would not be good for the case you mentioned. You'll be "cooking in someone else's kitchen" so to speak, and (at least in my experience) waste a lot of time just trying to figure out and undo what was already done to the theme.

"Most" of your theme's overall HTML is not going to be generated by Drupal really... you can code the bulk of your whole theme's layout without worrying at all about Drupal. After your own template is done, you will just plug in a number of snippets of PHP in the right places (see guide, and here's a list of the available variables), and only those spots will have Drupal-generated HTML (a lot of which you can control quite easily, and a little of which you can still control but requires a bit more effort... and in almost every case will have a Class and/or ID to latch onto with your CSS, so resorting to modifying the HTML output is rarely needed). There are actually only a few templates that contain nearly everything of core's theming (except in Views, which is another matter). Properly written Drupal 6 contributed modules also provide simple tpl.php files you can copy/paste into your theme's directory, and customize as needed. Drupal core, modules, etc will likely produce some IDs/Classes you don't need, but they can exist without hurting anything (if you don't custom style them, then the core CSS files usually still take care of it for the most basic things such as keeping core's jQuery/dynamic features and the basic essentials of the admin section working).

Given what you said I would suggest either:

1. Starting completely from scratch (no Drupal theme), designing the site "almost" like you would if Drupal were not part of the equation. I say "almost" because I suggest keeping a number of Drupal themes handy to refer to just a few key areas, such as menus, forms, etc. In some cases you will probably want to just go with the HTML output and the ID or Class Drupal gives in those cases, as it could require PHP overrides to change them. Most of the time though it doesn't matter. Just keep a theme or two open in your browser, and use Firebug to see the code of those areas (I'd recommend referring to the core or Acquia themes if possible so you know it's most likely done in a standard way, such as Garland, Acquia Marina, etc... or also refer to the core template files at example.com/modules/system). When your theme is done and installed in Drupal, as you browse the site it will become clear right away about any parts of the theme that are left unstyled (e.g. in the admin section especially)... identify those and either style them the way you want, or copy out the relevant bits from a different theme.
2. Alternately, choose a framework theme. I haven't tried them all myself (there are quite a few more now than when I started with Drupal), though Zen is often recommended. I think Zen "could" end up still being over-complicated if you want to do things your own way (Zen isn't "the right way" necessarily, it's just "a way"). Some other framework themes I've noticed include: Advanced Theme Construction Kit, Basic, beginningW2, Clean, and Framework.

Hope this helps :)

-- David
davidnewkerk.com | absolutecross.com
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I have had some fun playing around with genesis and basic