"This room is not bright."
I am also wondering this. I think it might be kind of a tense thing. One is negative and one is positive?
The adjective bright is akarui. To make it negative, you drop the i and add kunai. It works that way for most adjectives.
When negating い-adjectives, you drop the い and replace it with くない. For example, the negation of あつい is あつくない.