It may be easier to list what elements cannot be changed:
Majority of the site layout is fixed. The right hand side bar needs to have a minimum of 220px width, for in-house Ads. Since we design from 1024px resolution as the lowest common denominator, the left column is roughly about 730px-750px, taking margin/padding into consideration.
The whole content area background needs to be one solid background color, or background texture. Our html isn't set up for making the sidebar a separate bgcolor.
The only flexibility in layout is the header area, this includes the site logo and main navigation menu.
Everything else can be changed as far as visual treatment goes:
- Typography, websafe or font-face(as long as the webfont is free to use)
- Site logo
- Apple touch icon
- All Icons
- System message
- 404/captcha/error images
- Site background color/texture
- Content area background color/texture
- Question listing divider
- New user greeting box
- Ask Question help box
- Question owner background
- Accepted answer visual(on question listing pages)
- Chatroom box(sidebar)
- modal boxes(image upload, link)
- search box
- top utility bar
That's all I can think of now, I'm sure there are more. Basically if an element has a CSS class call, it can be styled(as long as it doesn't break the layout).
The Meta site will also receive a new theme too. The Meta theme typically is a desaturated version of the main site.
Upon graduation, we also update the designs elsewhere too, that are related to your site:
- Chatroom theme
- Twitter account avatar/page color scheme
- Email newsletter template
- Email newsletter sign up Ad banner
- In-house Ad design(no more Sketchy)
and further down the road:
- Moderator cards
- Generic site cards
- Swag for top users (T-shirts, stickers etc)
Hope this helps. I recommend browsing existing graduated SE2.0 sites, see what all can be done.