Stay with her. Sometimes, if your umbreon is newly evolved, it may take her a bit of time to adjust to the darkness because she herself is adjusting to the dark element within her, not to put it dramatically. She may even outgrow the fear over time on her own.
In the meantime, you can help her conserve energy by investing in a nightlight or lantern with adjustable brightness. Start the first few nights at the maximum brightness until she feels comfortable, then keep it on until she falls asleep. After a few nights, start off with the brightest setting, then slowly adjust it down a little bit. The next night, turn on the light and adjust it so that it reaches the amount of brightness you had achieved the previous night, then adjust it downward again. Keep going until your umbreon can tolerate having the light on at its dimmest setting. Then, the next night, don’t turn it on.
You can do this either in a pokémon center or in the outdoors, but I would recommend doing it in a pokémon center until she can get used to the darkness inside a building. Then, take her outside and repeat the process.
Additionally, do relaxation exercises with her before bed. Have her sit beside you and meditate. This can also help her control her inner darkness and hone her abilities until she feels more comfortable with them. Also, communicate with her during the day and at sunset to ask her what about the dark is so frightening. Although she might not be able to respond to you as children do, the act of asking her will allow her to think about it during meditation and face her fears on a psychological level.
Of course, don’t assume it’s not a valid concern, either. By asking her, she may be able to point out a clear danger as well. If you have a pokémon that can use Foresight, it may be a good idea to have that pokémon use it: if the technique doesn’t reveal any immediate dangers, it can at least put your umbreon’s mind at rest.
In short, helping any pokémon overcome their fears takes a bit of patience, anonymous, but in your case, it will take plenty of meditation and communication if it doesn’t simply resolve itself. Best of luck.