CSS adaption has no effect

Hi,
first of all – I love your plugin.

I’m trying to adapt the style of your lightbox with style.css in my child theme.
1) The only thing what works is removing the caption:
.slb_data_content {display: none;}

2) I tried to change the overlay color, with no success:
.slb_viewer_overlay {background-color: red;}

3) I tried to remove the padding and background color of the lightbox, with no success:
.slb_cotainer{ padding: 0; background-color: red;}

What am I doing wrong?

Thanks in advance.

https://wordpress.org/plugins/simple-lightbox/