How to create a horizontal menu?

Hi, if you go to my website (I have shared the link in the private section) and hover your cursor over Categories in the header menu, you'd see a long list of items. I need the category menu to appear horizontal like this - https://i.imgur.com/goifNfy.png

There are too many items so I would the list to appear in two parts like this - https://i.imgur.com/GDQsFCU.png

I will add more items in categories so the list should at some point look like this - https://i.imgur.com/goifNfy.png

I tried this CSS, didn't work - https://gist.github.com/generatepress/fc710ced4501566999fc

How do I create a horizontal menu?