some basic html/css design questions?

[rmr]

I'm having difficulty navigating the files in order to modify all the necessary html/css. Is there any literature with some basic instructions. I'm familiar with html and css, but I don't understand $hooks, etc. Any help would be greatly appreciated. I have been able to change a few things, but on the whole, I could use some guidance as to where things are located, etc.

Also, I tried to implement a different theme, but it doesn't show up in the "backend" when I upload it to my server...any reasons why?

Thanks