How to remove all image borders?

I use the theme ‘Twenty Twelve’ and want to remove the image borders of all the images. I googled a lot and already put the following code in all the CSS files, but that didn’t work.

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.entry-content img, .comment-content img, .widget img, img.header-image, .author-avatar img, img.wp-post-image {
border-radius: none !important;
box-shadow: none !important;
}
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Is there somebody who has the answer for me?