When you create a new project in vs17 (basic mvc .net web application in c#). It comes with bootstrap 3, which means all the scaffolded pages and designs are wrong since the
I ran into this same question when using a VS17 asp.net core mvc scaffold. With a little digging I was able to find a tutorial that walks you through updating bootstrap and jquery (Scroll about a third of the way down the page to the 'Upgrade Bootstrap Through Bower Package' section):
http://www.mytecbits.com/microsoft/dot-net/bootstrap-asp-net-core-mvc
Additionally, it is possible to create custom scaffolds with more modern versions of bootstrap. If you find yourself needing to update this often, it may be worth looking into that.