How to edit the hover behavior for pictures?

I would like to change the hover behavior on featured images that link to posts or subpage. They currently gray over with a thin plus symbol, and I would like it to be much more subtle.

Is there a tool for this that I am missing, or would have to adjust the code. I am okay adjusting the code myself, but I would appreciate a pointer as to where to find it.

http://www.cvillewinetour.com/cvilletravel/

Thank you,
Steve