ASP.NET MVC5 Bootstrap CSS Messing Up Navigation?

You're watching the course on ASP.NET MVC 5 made by Mosh Hamedani I presume. You're downloading the latest version of the Bootswatch theme which is meant for Bootstrap 4. The default MVC 5 template uses Bootstrap 3.

You need to download this version of the Lumen theme: "https://bootswatch.com/3/lumen/"

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Alternatively you could upgrade to Bootstrap 4 by updating your Bootstrap NuGet package.