CSS for the slider’s transparency background

Hello,

I absolutely love the slider. It was one of the reasons I actually chose this theme ??
(and you should have my PayPal donation already! Continue the great work!)

Now, I’d like to have the darkened area of the slider (where the header and abstract as well as the blue action button is placed) configured. Actually I’d like to know, how and where to place code for the CSS to make the background even more darker.
A small code snippet for the child theme’s css.php would be perfect.

Many thanks in advance for any hint!