There are many creative things you can do in order to make your lucid dreams brighter. First of all, do not believe that just because you are wearing a sleep mask, your dreams will be darker. This isn't true, the mind is powerful and you can make your dreams as bright as you want.

Here are some things I recommend you try:

-Assume you are wearing sunglasses in the dream and that is why it is so dark. Reach up to your face and take them off.
-Look for the sun/moon. Point to it and command it to be brighter.
-Find a remote that controls your dreams brightness.
-Pretend there is just a temporary power outage in the dream. Find a power box or electrician and make it bright again!

Good luck