Krodium follows your Mac's appearance until you choose otherwise. When your Mac changes between light and dark, Krodium changes with it.
To choose for yourself, use the Light and Dark switch in the toolbar. Krodium then stays on the one you picked.
If the switch is not in your toolbar, you can add it. See Customise the toolbar (krodium.com/help/customise).
Still stuck? Write to [email protected].