When I use the spacebar to use Markup to crop photos, the photo becomes darker. How do I change the setting?

I'm not sure what you are doing. Markup is used to add text and figures to a picture, not for cropping. The spacebar doesn't take you to Markup, it opens a selected picture in Full Screen Preview mode.

When I crop a picture, I go into Edit mode by selecting the picture and hitting the RETURN button. At the top I get this menu:

And I select "Crop."

I don't see a change in brightness when I crop a picture.

Can you be specific about your actual button and menu sequence?