Unclear text

The text in the content area, is some unclear when I use some themes, and sharp when I use other themes (usinng IE9 / latest version for Vista).
For instance, if using theme Suffusion, theme Childishly Simple, the text in the content area is not so sharp in IE9.
Changing to another theme, for instance Twenty Twelve, the text in the content area get sharp again.

Do anyone know, what is the reason for it, and how one can change the text to to be more sharp/clear if using a theme where the text get less sharp?
It’s really not comfortable to read the text.