Hello,
We have checked your site on mobile http://prntscr.com/l7x2dh As you can see, no problem. When you resize the window update the page and you will see.
If you want to change the background on the scroll, try this code:
body.basel-header-overlap .header-simple.main-header.act-scroll {
background-color: white;
color: black;
}
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .main-nav .menu>li.current-menu-item>a,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .main-nav .menu>li>a,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .right-column a,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .amount,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .main-nav .menu>li.menu-item-has-children>a:after{
color: black;
}
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .right-column .search-button a:after,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .right-column .search-button a:before,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .right-column .basel-burger,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .right-column .basel-burger:after,
body.basel-header-overlap .color-scheme-light.header-simple.main-header.act-scroll .right-column .basel-burger:before{
background-color: black;
}
Set the color as per your needs and add the code to the Theme Settings > Custom CSS.
Best Regards