If anyone is still interested in this, I did that today (scratching a personal itch - not knowing this had been filed before).  

What I have done:
1. Moved all the font/color information to a separate css file
2. Used html5 syntax.
3. Recreated a css style sheet that approximate the current look
4. Created a new, very simple style sheet, that is used as a default. It could stand to be greatly improved upon.
5. Checked the validity (W3C) of a sample of the pages generated. This required me to include some empty <dt> and <dd> tags.
6. Added an option to start the server with an optional, user-defined css stylesheet.
7. Made some small unrelated edit (adding spaces after commas, removing
extra spaces before commas, etc.) to reduce the amount of noise from my linter.

I implemented this as a separate project; I did not attempt to run any existing unit tests, nor create new ones.  Other than for the additional option (user defined style sheet), I tried to avoid making any change to the functionality. The styling possible to do is thus limited as I mostly replaced existing strings/templates by new ones.

My implementation can be found at https://github.com/aroberge/mod_pydoc
(sorry, not an hg repo).  I did not include a license; I took the existing pydoc without asking permission. It can be understood to be under the original license.