Very simply, I want to take a current theme and modify it to create a new theme, or to do it from stratch. Modify a current one is my preffered method.

I want to change the logo, the colors, and just about everything else. How do I do this?

I have been fiddling with the template manager, but that just seems to modify the blocks. I dont want to modify the block content... just what the borders look like, the font size and such.

How do I do this? Which CSS do I need to modify?

How do I get it to use my logo graphic and my own icons for things such as New Topic in the forum and such?

Thanks

Take a look at the faq, How do I build a theme.