I do not load styles when starting a server in flask

I do not load the css styles of my page when I start it with flask, but I open the file on my own if I upload them.

This is my flask code:

from flask import Flask
from flask import render_template

app = Flask(__name__)

@app.route('/')
def index():
    return render_template('index.html')

@app.route('/microfonos_para_cantar')
def microfonos_para_cantar():
    return render_template('microfonosCanto.html')

if __name__ == '__main__':
    app.run()