There is a separate light bulb that lights the outside temperature readout. If this is burned out it will be hard to read the display. One way to check if the LED readout is still good is by shining a light in front of it and you should be able to read it. Also, if you can read it with direct sun coming in the window the LED readout is good and it is just the bulb.

To replace the bulb you need to remove the instrument cluster.