The directory could not read my css file although I put in the static directory for Flask

/205CDE Web Project
    -app.py
    /templates
     - mainpage.html
    /static
     - mainpage.css

mainpage.html

<link rel="stylesheet" href="">

app.py

app = Flask(__name__)
@app.route('/mainpage')
def foodweb():
    return render_template('mainpage.html')

I had a problem that although I put my css file into static folder, the html template file still cannot recognize the css file. May I ask is there a trick that I missed so the template could not update?