If you previously used Duster on the blog in question then you can select it again here > Apearance > Themes and use it again on the same blog by typing in the theme name “duster” http://theme.wordpress.com/retired/duster/
Did you remove the CSS customizations when you switched to Twenty Eleven or not?
When you alter a theme’s stylesheet, you only need to put in the Editor the modification/additions to the CSS, not the entire thing. 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.