I'd like to add on to what Alpha said. If copy/pasting those codes doesn't help/if you have the original game without any mods, the manes do seem to be a lighter colour at night than in the day.
Not really "glowing" but they're out of sync with the pelt colour at night, whereas they look fine in the day.
The only true solution to this without modifying anything is to darken your mane colour. For example: I had a character with a pure white mane (255 255 255) and it looked like a self-illuminated lightbulb at night, so I darkened it to a silver/white colour (200 200 200) and that helped a lot. You don't have to darken it so much, just try to edit and play around with the colour.
If not, there's mods out there to make your character more "realistic". They typically set an ambient shade on the mane so it no longer "glows".