i found paddy green theme work the best for my in my project.
the only thing that i have problems with is changing the way the submenu appears.
currently, when an item in main menu has subitems, the submenu appears on the right side, in that main content section.
I would prefer to display the submenu immediately below the main menu, in the horizontal form.
Would this be possible through .css and .ss changes or it requires more work ?
If you are talking about a drop down menu, I am sure it is possible but requires more work for browser compatibility.
I am sure you can get the regular submenu to be below the main menu, by adjusting the float properties on the main content and the submenu. - You will also need to change the order in which they are displayed in the page.ss file.
Download firebug if you haven't already. It works for firefox browser, and it lets you fiddle with the css and experiment without making any real changes. I use it all the time to highlight an element and see what css it is using (it tells you what line in what css file to change).
I am also using the paddy green theme in my recent project.