Cannot center large header slideshow

Thanks in advance from a WYSIWYG guy who knows almost nothing about code… I used the Easing Slider plugin to create a header slider for our site (I’m experimenting on a test server of ours, nothing is there except the slider).

https://richmondmediatestserver2.info/rm/

The slider is 1680 pixels wide, and I can’t center it. I tried centering <div class=”header”> in header.php: <div class=”header” align=”center”> but it didn’t work, But strangely enough, when I aligned it to the right: <div class=”header” align=”right”> it DID work, but I don’t want that.

I’d like it to center like this WordPress site:
https://www.dnd.fr/

Ideas? Thanks!