In reading "Pro Asp.net core mvc 2", the bower management package is used, which may be due to changes in vs2017 or bootstrap. Follow the steps, and there is no corresponding style at all. At first I thought it was the browser inconsistency, but the problem is still the same after changing the browser. I just thought that it might be a problem with the package. Forgive me as a novice for not knowing anything. I searched online and found it true. Bower has newly created a bower_contents to store the corresponding files, which is not under wwwroot. I moved the file to wwwroot again and recompiled it.
For the record, there may be other newbies who also encounter this problem.