Add this line.site-nav li details#site-nav-item--6 summary { color:red !important; }

your menu is too big and it will scroll if you align in one line

I have understand it should be like that

Please click on Customise button and follow the steps 1. Click on header    2. go to custom css and past that code.   3. #site-nav-item--6{ color:red }It will work 

You need to toggle this id shopify-section-sections--17005268730034__navigation_1_nFW3GmHide in mobile and show in desktop 

You need to hide desktop menu in mobile view because in the mobile menu is already working. 

Hi Please add theme.liquid below line #site-nav-item--6{ color:red }