Change hover color menu Twenty Eleven

let’s see your site

Close off this style:

img[class*="align"],
img[class*="wp-image-"],
#content .gallery .gallery-icon img {/* Add fancy borders to all WordPress-added images but not things like badges and icons and the like */
	border: 0px solid #ddd;
	padding: 6px;
	max-width: 97.5%;

Look at this image outlining the syntax of CSS: http://www.w3schools.com/css/selector.gif , particularly of the curly braces.

Instead do this:

img[class*="align"],
img[class*="wp-image-"],
#content .gallery .gallery-icon img {/* Add fancy borders to all WordPress-added images but not things like badges and icons and the like */
	border: 0px solid #ddd;
	padding: 6px;
	max-width: 97.5%;
}

Triple-check you did it, because it’s not showing your change: http://www.namtok.nl/wp-content/themes/twentyeleven-child/style.css

Yes if there’s an accident like that, if there’s a syntax error, then all of the code after it will just be ignored.

A syntax error can break your CSS so it’s worth looking up on the syntax https://developer.mozilla.org/en-US/docs/Web/CSS/Syntax