You need to have a ‘static’ folder setup (for css/js files) unless you specifically override it during Flask initialization. I am assuming you did not override it.
Your directory structure for css should be like:
/app
- app_runner.py
/services
- app.py
/templates
- mainpage.html
/static
/styles
- mainpage.css
Notice that your /styles directory should be under /static
Then, do this
<link rel= "stylesheet" type= "text/css" href= "https://stackoverflow.com/questions/22259847/{{ url_for("static',filename="styles/mainpage.css") }}">
Flask will now look for the css file under static/styles/mainpage.css