Maybe a template sensor over the openweathermap_forecast_condition state? You can pretty easily have a template change its icon, and mdi: has all the icons built in.

Here’s a template I use for current condition in a card, but it should work similarly for the forecast. Or works just need to be applied to the icon of a template sensor.

{{ iif(
        is_state("weather.openweathermap", "partlycloudy"), "mdi:weather-partly-cloudy", 
        iif(is_state("weather.openweathermap", "clear-night"), "mdi:weather-night",
        iif(is_state("weather.openweathermap", "exceptional"), "mdi:exclamation-thick",
        "mdi:weather-" + states("weather.openweathermap"
        )))) }}