AI, which isn't guaranteed 100% accurate, offers this...
"NCH PhotoPad does not currently offer a dedicated interface (UI) theme setting to switch the entire program from black (dark mode) to white (light mode) or vice versa. Instead, PhotoPad allows users to change the color of the photo background layer within the editing canvas to white, black, or any custom color using the Background tools, but this does not affect the appearance of the actual software interface or menus—only the editing workspace for specific photos.
UI Theme Customization Details
Users can change the background layer in their project to white by selecting the "Solid Color Background" and picking white, affecting only the visible workspace behind or around an image with transparency.
PhotoPad's menus, toolbars, and interface colors remain set by the application and cannot be changed to a light or dark application theme via program settings as of the latest release.
If true UI theming becomes available in a future version, it would appear as an option within the program’s settings or preferences menu, which is not present in the current documentation or tutorials.
Editing Background Versus UI Theme
Canvas background color: Can be set to white, black, or any color for each project.
Application appearance: Not user-customizable between light and dark modes as of October 2025.