Html overiding
For a single page, you can create whatever html you like with drupal, using
- templates
- theming functions
- Write the exact html yourself, creating a theming function or just putting it in a template while.
Since you want a unique layout you want to look into a custom page.tpl.php. You can create a page.tpl.php for any url, which will allow you to modify the entire page html for that page, or just tweak it here and there.
Drupal has a quite flexible theming system, so there are many ways to change the markup. All of this stuff would need to go into your theme or a module.
CSS overriding
You will still have the css files that are used in the drupal site.
- You can add additional stylesheets, with drupal_add_css(), but you might have problems with the other style sheets conflicting.
- You can remove all other stylesheets in template.php or remove them from your page.tpl.php, but then drupal_add_css wont work either.
- If you do this, you could generate the link to the css file yourself, in
- your markup
- template.php
- the page.tpl.php file.
Static page
If your aim is to create one on only on page, with different layout ect, the quickest and easiest way, would be to create our own page.tpl.php file, and just write all your html in there and forget what Drupal can do. It's not generally a best practice, but in this case, doing stuff with the flexibility of Drupal might be a bit overkill, especially if you don't know Drupal well. You could possibly with the page template naming convention do this aproach, just adding a file with your markup in your theme. This approach would be good for a static site, but can also be done with a bit more effort for other pages. If you want to use a lot of modules, this wont be the solution for you. You should instead look at the theming functions / template.php and what can be done there.
Change your Drupal backend / admin theme
If your aim is to create a backend theme, drupal has several modules for this, the most used these days is probably the admin module. Even if this is not your aim, this is a great module to make your backend a bit more user friendly.