Most probably this is related to the error we were experiencing yesterday wherein the main.css wasn’t being added for some reason, the actual error was related to PostCSS and Autoprefixer versions not matching. It has been rectified now.
Here is the forum link to the problem: 404 error on main.css because of different postcss version requirements
And this is the actual issue: https://github.com/ionic-team/ionic-app-scripts/issues/1142
As you are new so don’t worry about all this, the problem has been rectified and I believe if you create a project afresh now it should work properly.