Application not picking up .css file (flask/python)

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= "{{ url_for('static',filename='styles/mainpage.css') }}">

Flask will now look for the css file under static/styles/mainpage.css

In jinja2 templates (which flask uses), use

href="{{ url_for('static', filename='mainpage.css')}}"

The static files are usually in the static folder, though, unless configured otherwise.