Custom description in imagebrowser

I want to place the description of an image in imagebrowser template on the center.
So I went to Gallery, Other options, Styles, Show customization options, and change the nggallery.css as follow:

/*
CSS Name: Default Styles
Description: NextGEN Default Gallery Stylesheet
Author: Photocrati Media
Version: 2.12

This stylesheet is provided to allow users the ability of overriding the default styles for all display types
*/

h3 {
font-size:large;
text-align:center;
}

but the text align doesn’t change. I’ve also tried to change line 4 of the file nextgen_basic_imagebrowser.php as follow:

<h3 style="text-align:center"><?php echo esc_attr($image->alttext); ?></h3>

No way.
Could you help me, please?
Thank you.