I created a new MVC application in VS2019 using .NET 4.7.2 and MVC 5. The template renders fine.
https://i.stack.imgur.com/r6XfF.jpg
But... when I have downloaded the Bootswatch Lumen theme and then updated the CSS file referenced in the App_Start/BundleConfig.cs file and then rebuild and refresh, the menu at the top renders as the Application Title with an empty button which expands the menu items.
https://i.stack.imgur.com/XmzNs.jpg
https://i.stack.imgur.com/cKcJb.jpg
This is not what happens in the tutorials. The menu items remain as a menu bar at the top and it looks like a basic theme change only.
Apologies if this is an easy one (I hope it is!) I'm a veteran WebFormer of 12 years finally making the MVC jump.