However, if you must stick to CS6 (for legacy plugins, specific ActionScript 2.0 needs, or preference), will give you the visual dark mode you want, provided you remember to toggle it off before exporting your final animations.
In the deep corners of DeviantArt and GitHub, enthusiasts have created custom .dll and .dat skin files for Flash CS6. This method actually changes the UI chrome (the borders, buttons, and toolbars) to dark grey or charcoal. adobe flash cs6 dark mode