Clocks in the palette can use either dark text on a light background or light text on a dark background.
On macOS 13 or later, choose World Clock Deluxe > Settings. On macOS 12 or earlier, choose World Clock Deluxe > Preferences. Then click Palette.
Select or deselect the "Light text on dark background" checkbox.
Control-click the palette's background and choose "Light Text on Dark Background" or "Dark Text on Light Background" from the shortcut menu.