Why is it on a tv show, the lighting seems to change over the years?

it seems as if on a tv show, in the beginning, they use different lighting on the studio set. i will use a certain tv show for example, beverly hills 90210, in the beggining of the show in 1990, it seemed the lighting was darker, then when the show ended in 2000, the lighting was lighter and ive seen this with a few tv shows, such as the cosby show and cheers.