Remove colors from image_gallery.css

It is my opinion that the image_gallery.css file for this module, should be completely devoid of any color selections.

The reason for this is simple, yet seems to almost always be overlooked. When adding this module in its current state to a Drupal site, it has the potential to render itself unnavigable, and requires the webmaster to track down the offending piece of styling that is the cause of the issue.

Leaving the colour selections out of the module makes sense for two reasons.
1. By leaving the color definitions out, the module will, by default blend into the sites theme.
2. By still utilizing a css file for the module itself, it gives webmasters/site designers the option of styling the image gallery differently from the default site theme.