PrusaSlicer supports dark mode on OSX and Linux natively. If you have dark mode enabled in your system settings, PrusaSlicer should automatically start in dark mode as well.
You can enable dark-mode manually from Preferences - GUI - Use dark color mode (experimental).
Implementing dark mode for Windows took significantly more effort than we expected. Unlike OSX and Linux, Windows has no official dark mode for Win32 controls, which our multi-platform GUI library (wxWidgets) uses. So we embarked on a journey to implement it ourselves. We started skinning the existing Win32 components and implementing replacements for those, which were not skinnable. Luckily, we discovered Notepad++! As far as we’re aware, Notepad++ is the only Win32 open-source application that implements Dark Mode to a reasonable extent. We ended up using some of the source code from Notepad++ interfacing with the unpublished Dark Mode API, which is based on this project.