You are lucky that the leaves are evenly dark, because if they had small dark spots on them, this could be a bacterial or fungal infection.
I agree with the previous response, the tree might need it's soil to be changed. This isn't urgent, as the problem is not very serious.
Another reason that lemon tree leaves may be so dark is beacuse it is not getting enough magnesium. Look for fertilizer that has magnesium and fertilize with it once in a while, just to upkeep the lemon tree's well being