The "Stage" is usually the brightest part of your screen. You can change this to a darker neutral color to simulate a dark workspace.
If you must use Flash CS6 for legacy projects, consider these external solutions to help with the light UI: adobe flash cs6 dark mode
Click the Stage Color swatch and select a dark gray (#333333) or black. The "Stage" is usually the brightest part of your screen
Use software like f.lux to warm up your screen colors at night. adobe flash cs6 dark mode