One opposite of "bright" is "dim." Dim lights and dim places are just a little bit light. They're not totally dark, but they're not bright, either. In other words, a dim place is slightly dark, with just a teeny amount of light to see.
But let's end on a happy note! You could say that your excitement for the weekend is undimmed, even though you don't have fun plans. Or you could say that old age hasn't dimmed your grandmother's beauty. Something that stays bright, lovely, or happy over time is undimmed.