I have a similar issue but the above (page width 100% - post container 70%) didn't work.
What I'd like to see is to have the header and footer at 100% accross the screen but the middle part to be fixed width (say 800px).
I've tried
1. td#middle {width: 400px;} - not working
2. adjust ATO/Centre coloumn - not working
3. adjust ATO/Post/pages - not working
I noticed that in the CSS.php (I know, didn't touch it, just looked

) that td#middle has 100% twice - could this be the issue?
I need to emphasise that it's not the entire container width I want to change just below header above footer area. On a normal page I would have a container div that includes left sidebar, content, right sidebar - but I can't find this here.
Many thanks!
ATA 353
http://79.170.44.110/westaucklandparents.org.nz/
(I keep working on the page so not sure what stage exactly it'll be if you're looking at it!)