The ThemeInstallTask expects an existing Theme configuration within the module's bootstrap files.

 new BootstrapSingleResource("", "", "/mgnl-bootstrap/theme-" + themeName + "/config.modules.standard-templating-kit.config.themes." + themeName + ".xml", ImportUUIDBehavior.IMPORT_UUID_COLLISION_REPLACE_EXISTING).execute(ctx);

When creating a new theme, there can't exist a theme configuration yet.
This is one of the most often stumbles for people when creating a custom theme.

The ThemeInstallTask should use a custom task, which first checks if the resource exists within the jar/module.