Darken slider images

I’m trying to darken the header slider images that I have on my website even more, but I’m not finding it in the theme template.

How can I do it? CSS?

In other words, I can’t find where to increase that darkening.

The problem I have is that I cannot identify the slider in the template stylesheet.

Although there is a section dedicated to the slider in the style sheet, I have tried to modify the transparency fields of the photos (the theme, by default, already has a certain degree of opacity in it), but I can’t get it to take effect.

What I can do?.

Thank you!

The page I need help with: [log in to see the link]