Am currently new to ASP.NET and am currently in process of learning how to change theme.
What I've done:
With VS2017, I started a new ASP.NET Web Application. This is the default theme before i changed the CSS.

I got a theme from : https://bootswatch.com/lumen/ and replaced Content/bootstrap.css with the new theme code as ive been instructed to by multiple tutorials.
After doing those steps, my header navigation messes up and suddenly converts into a ugly looking dropdown menu?
Below is what it should actually look like.

Could someone tell me where I went wrong, or if ive missed any steps? Im following instructions from a video tutorial to the T and ive not missed any steps and am unable to achieve the same output. The only difference is the instructor is using VS2013.
