Hero Image Covers Main Menu

Hello,

This is my site: https://swiftracingjapan.com/bont/safety/

And Hero is covering the primary menu.
I’ve looked into the past thread and tried with:

#header {
z-index: 100;
}

or

.site-header {
z-index: 101;
}

Neither of them works…
Is there any other way to fix this?

Thank you for your help!
Rika