Your question isn't really a Nabble issue, but one that should be directed at the support service for your web creation software. However, I will make one observation about your main question...
Using "position: absolute" is rarely the best way of locating a major part of your page's content. It takes that content out of the page's "normal flow" and should, for example, your visitors use browser settings to magnify text or images then your page could become seriously corrupted with the features such as your main navigation menu becoming hidden behind the absolutely positioned block.
Also, I am not sure what you mean by "The CSS examples that this web site is presenting seem really vague". Does "this web site" referr to your Nabble forum?
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.