Navigation

Navigation in WordPress themes can be set by going to appearance -> menus.

By default in the theme navigation submenu is displayed in two columns, but you can set it to single column by going to parent element and adding class "column-single".

I don't have field to add css class to navigation, what to do?

This can happen on new WordPress installations. On the top right of the screen click on the button Screen Options, and then enable adding css classes.