What is the most basic theme available

Trying to get my head around php and css.
I copied some code into my header to try something out and had to keep fiddling to get it to work.
So, as a newbie, what theme is out there which is very basic and has a very basic style sheet, so that I can look through it and start to make sense of the reason for so many entries in the style sheet and get an understanding of header.php etc.
I found it so much easier under html and find it embarrassing that css blinds me.
And to finish with a naive comment, I know css is flexible, but cannot believe you need so many entries where a table in html, for example, with a bit of justification requires so little.
Cheers
John