There is no way to actually make it night all the time, but you can make the day be as dark as night.
Simply select 'Day (bottom)' and change the color to a nighty color, then do the same with 'Day (top)', choosing a slighly different color if you want. Then repeat it with Dawn (bottom and top), Dusk (bottom and top) and night, if you want to change it's colors too.
Then scrolling down a bit you'll find Sunrise, sun, sunset and moon. Maybe you could change the colors of the 3 suns to the same color of the night, so they won't appear.
Then you'll find Dawn, day, dusk and night light. Change their colors to all nighty.
Then you just need to set the colors of the clouds!
What a long explanation... I hope you understand. If not, you can ask!
And you don't need to export the sky to use it in your map. You just need to save it then load it in your map. But make sure you write just 'YourSkyName' not 'YourSkyName.sky'. It won't work if you add .sky XD
And when you add it to your map, it will only appear when you test, so don't worry if it don't appear right away!
Oh, and I'm sorry, but I don't know how to make weather exactly...