Edit Transparency Background

I know that right now I have a black background set for transparency with a custom image background behind it. Is there a way to darken or lighten the transparency setting on the black piece behind the main content? I'd like to either make it darker or make it complete solid black (while maintaining the custom background behind it all). Any suggestions on where to go and edit?

www.w3schools.com/Css/css_image_transparency.asp

Thanks, I knew I had to adjust the opacity of the transparent image but I cannot find the location in my files to edit the transparency. What file of the sportal is it?

Image path is:

templates/hot_sportal/images/bg_transparent_black.png

Css:

templates/hot_sportal/css/layout.css

.wrapper_outside{...

Ivan