If your target browser is IE7 I would throw out all css3 and keep it as simple as you possibly can.  If you're replacing divs with tables (I certainly don't envy you...) I'm not sure adapting an existing theme would be much help (other than possibly educating you in the elgg views system) as there would be so much to change you might as well start fresh.

@rjcalifornia I'm a little scared to implement too many hacks since I feel the theme would be a little unstable and difficult to modify later. Since most of my clients love to tweak with my work after the delivery (quite common in webdesign, I know) I prefer to keep it very simple to edit again and again until this client is fully satisfied.
For this reason I think I'll stay with the suggestion from @Matt for a fresh start and a very easy structure. To work I think I'll go back to Adobe Dreamweaver which some years ago was for me the main tool to develop tables and avoid madness, considering the huge amount of TLs, TDs, COLSPAN and all their happy family.

Anyway, you could confirm that elgg could be integrated in tables as well in DIVs and I'll not discover some huge problem later? For my experience with CMSs, it should be absolutely possible but the right time (and the only one) to have this kind of doubt is before beginning :)

Do you know of any good tutorial I could use? I have sleeping accounts with Lynda.com and Tuts+ that I use sometime to catch up with some technologies but I didn't find anything on eelg.

thanks for your help!

Enrico

It definitely can be done.  I've never used dreamweaver, I think it's more of a graphical drag & drop kind of tool - not sure how useful it would be with the views system (which if you're not familiar with Elgg you should read up on before you dive in)

http://docs.elgg.org/wiki/Engine/Views