Check Edit > Preferences > User Interface (Windows) or Flash > Preferences > User Interface (Mac) to see if brightness sliders exist.
You might wonder, "Why not just use Adobe Animate CC? It has a native dark mode toggle under Edit > Preferences > Interface." adobe flash cs6 dark mode link
Use modern tools alongside Flash
For Windows users, Adobe CS6 applications read their UI colors from the Windows Registry. There is no toggle, but you can manually edit the color values. Many forums share ".reg" download links that automate this. Warning: Always back up your registry before applying third-party links. Check Edit > Preferences > User Interface (Windows)
The Flash community, specifically the passionate users on DeviantArt (back in the day) and GitHub , created custom .dll and .dat skin files that override the UI. The most famous is the or the "Obsidian" skins. There is no toggle, but you can manually