Dark Sky sensor icons seems to be missing. Only text is shown

I have configured to use Dark Sky as provider for the outside weather. However, I cannot see any icon, only writing. It seems like home assistant is missing the icons to show the weather.

I have installed Home Assistant on a raspberry pi 3b with DietPi as distro.

Capture

Do you know why this happens?

This is my sensors part in my configuration
# Sensors
sensor:
# Weather prediction
- platform: darksky
api_key: xxxxxxxxxxxxxxxxxxxxxxxxxx
monitored_conditions:
- icon
- temperature