Amygail, thanks for the try but it is not working for me or may be i have entered in the wrong style sheet. Which style sheet do i have to enter this. in powerblog style sheet or thematic style.
You always add your CSS modifications to the childtheme’s stylesheet. Why? Because your childtheme’s stylesheet is loaded after the thematic stylesheet is loaded, which means that all CSS will be overruled/overwritten by the stylesheet which is loaded last (which is your childtheme’s stylesheet).
Maybe you are looking at a cached version of your CSS, make sure you do a proper reload in your browser!
Beside, amygail’s solution should work if you edited it to the correct slug name.