This is what I see on the footer of the blog linked to your username:
Blog at WordPress.com. Theme: Customized Mystique by digitalnature.
This is what I see on the footer of the blog linked to automatically by the software in the OP:
Blog at WordPress.com. Theme: Customized Twenty Eleven
This is what I know:
CSS is theme specific. If you have custom CSS and later change themes, you need to delete whatever CSS you have in the Editor, otherwise, you’ll get undesired results.
To ‘reset’ a theme, you need to delete the CSS in the Editor, make sure the “add to existing CSS” radio button is checked, then save. Things should go back to normal.