Changing Lightbox opacity in Matsuya

I'm using the Matsuya template and have an issue with the opacity of the lightbox viewer. Currently there's no way to make it fully opaque (white), and none of the CSS I've tried has worked. Does anyone have any ideas? 

Can you share link to your site?

https://badger-tarantula-aerk.squarespace.com

thank you so much tuanphan - we'll try that today!

Hi Tuanphan - it worked!!!! - many thanks for your help

it worked great! thanks so much