I have done quite a few websites and have no problem editing HTML and CSS.

Now I tried to edit WP theme "Thematic".

When I look at the template files, they are all php and a few css files.

There is little HTML in the php files, and there is no central file that shows the whole document.

It's like trying to read a novel with all the pages flying around, and no context.

When I look at the blog through Firebug, I can see the html.

But I can't edit it.

Where is the Wordpress 3 HTML so I can edit it?

Nobody ever created a child theme here?

Thematic is a mostly PHP. If you want to edit it directly you'll need to edit that PHP. However the idea is to create a child theme instead of editing the framework itself.

In the child theme you can overwrite any of the framework files. The Codex has a page on child themes to get you started.

Also Ian Steward, the person who created Thematic, has written plenty of tutorials on his site for developing child themes. Here's one covering child theme basics.

When you view pages through Firebug you're seeing the html that's output after the php engine has parsed all the php code and converted it to html. Comparing the output html and the original files with php is actually a good way to begin learning php. I know it looks like a foreign language at first, but there should be enough html in those php files to help you find what php is producing what html.

But php files are only a few lines long.

Where do all the divs and classes come from?

Created by the scripts, you can generate a lot of HTML in one function call.

Just like Chris said a line or two of php can put out a lot of html.

I haven't worked with thematic a lot recently, but to you should be able to customize it a lot solely through the css file of a child theme. Use the html you see in Firebug to guide you and style that output source code.

Thematic comes with several different layout stylesheets too so to do something like move the sidebar from the right side to the left it's just a matter of including a different layout css file. Look in the library folder for the different layouts you can get and then look in the css file of the included child theme to see how they get included.

You can modify more of the theme through the functions.php file, but that will require php coding. Mostly you're going to want to create your own css file and use the rendered html source you see by viewing the source directly. The classes and ids and general html you see by viewing the source are all things you can control through the css file.