In Google Docs, go to Settings -> Document Settings, then go to the Editing tab. Make sure that "Create new text documents using the latest version of the document editor" is un-checked. They don't provide a way to convert new-style documents to legacy-style documents, so unfortunately you'll need to create a new document after unchecking the "latest version of the editor" box and copy any new-style document that you want to do serious formatting on into that new document.
Google's official statement is thus:
"These features from the previous version of Google documents, however, won't be available in the new version:
* Offline document access via Google Gears
* Edit HTML
* Edit CSS"
As noted above, the Google help forums are buzzing with "bring back the ability to edit HTML/CSS, you jerks!" topics. It would probably be a good idea to add your voice to that if the feature is important to you.