It may happen that the AdSense ads don’t fit very well in your beloved WordPress theme, before to fall in frustration please check out these two articles, they could help you to improve the user experience of your visitors.
The first article, written by Amit Agarwal describes two ways to make more responsive your ads.
The second one instead, written by Kevin Craighead , suggest to modify the CSS file. It deserves to be read.
Please remember to handle with care the CSS file of your theme.