Currently, the Deltachat forums lack the option to have the Light/Dark theme depend on system preference.
This is, to my knowledge, usually the default Discourse configuration, but that default might have changed since this forum was first set up a few years ago. It would however be nice, as it means users do not have to constantly go back into the settings if they wish to have the theme match their current device’s theme.
(An explicit “Light” theme preference would also be required, as “Default” would be dynamic following this)