Personally I prefer to develop our own CSS and honestly I don't like so much the default Theme (I on't have time to see others).
More over I think it takes too much time too analyze and adapt another CSS to our needs (always IMHO).
My proposal is....
1. Layout migration section by section.
Identifying the sections and pages that will most benefit from the
conversion will simplify the whole work.
I think that as a good start we should focus our attention to the content, fixing headings and paragraphs, removing <br> and <b> (search for <b>ad and <br>eakfast ;-) , replacing where possible tables with div/lists, paddings and margins.
Cleaning the base html tags styles.
2. Logical divisions of content.
This should the second logical step. We should identify our pages for logical divisions of content.
I mean something like:
- Main menu
- Login/registration menu
- Option/languages menu
- Headers and footers
- Content
- Related information
- Others...
And start to replace these with divs and lists (ul) where possible and/or fixing existing tables.
3. Look and feel.
When the content will be more organized and clean we can start to think about the look.
- Colors.
- Header style.
- Menus positioning.
- and so on...
All of these always taking care of usability and accessibility (feeling)...
What do you think?
Cheers
Antenore.