Customize Google AdSense ads from your account dashboard, this is upcoming feature of Google adsense. But you have to wait until this feature is enabled on your adsense account.
"This new ad management feature means that your ad unit settings (such as colors and channels) for new AdSense for content ad units will be saved in your AdSense account every time you generate ad code. Then, if you'd like to change any of these settings in the future, all you do is make the update within your account -- you'll no longer need to manually replace the ad code on all of your pages. For instance, you can quickly change the borders of all your 300x250 medium rectangles from red to blue with just a few mouse clicks. Fancy! We hope that this new feature will help you save time and will simplify the process of optimizing your ad units."
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