Toggles EditPad’s user interface (toolbars, menus, dialog boxes, etc.) between the default Windows theme (which is a light theme) and EditPad’s own dark theme.
If you regularly switch between themes you should select color palettes that have companion palettes on the Colors and Syntax page in the file type configuration. Then switching between themes also switches the file types between light and dark palettes. Switching between themes also swaps the text and background colors on the Panels page in the Preferences to match the theme if one color is very dark and the other color is very light.