first you have to learn to make child themes
The reason is, twentyten is the default theme to WP, so whenever you upgrade WP, twentyten is overwritten.
So without a child theme in place, your changes will be lost.
You can make a very basic child theme as that tutorial shows, and just use the import rule to call to twentytens css, or you can copy all of the style.css to your child theme
margin: 0 0 18px 0;
margin: 14px 0 18px 0;
This bit of code is what controls the title and tagline. You could change fonts here as well as placement by adjusting margins
As for your BG image, isn't there an option when uploading your BG image to not have it repeat?