Levels

The dialog for adjusting levels can be started by clicking on "Colors -> Levels" on the user menu.

You can do one of the following:

To increase brightness of the light colors and contrast, move the "Input Level" marker (white) to the left. The marker showing the level of intermediate colors (between dark and light) will shift as well. Adjust its position manually if needed.
To make dark colors darker, move marker "Input Levels" (black) to the right. The marker showing the level of intermediate colors will shift as well. Adjust its position manually if needed.
To adjust the level of intermediate colors independently, use the middle marker "Input Levels".
To reduce contrast of an image and lighten it, move the "Output Levels" marker (black) to the right.
To reduce contrast and darken an image, move the white marker "Output Levels" to the left.