Remember that this changes the actual background color of your animation. If you intend for your final animation to have a white background, you must switch this back to white before you export/publish your movie. Treat this as a "view mode" while drawing.
The Timeline, Library, and Properties panels are separate .swf files. You need a tool like JPEXS Free Flash Decompiler :
Adobe Flash CS6, released in 2012, was designed during a time when dark mode wasn't a widely adopted concept. As a result, the software's default interface is light-themed, which can be harsh on the eyes. Furthermore, Adobe Flash CS6 has been discontinued, and no official updates or patches have been released since 2015. This has led to compatibility issues and limitations, making it challenging for users to customize the interface.