NoActiveThemeException is triggered it's because there is no active theme selected.
To select an active theme, you have at least 2 choices.
If you are using the HarmonyAdminBundle, you just need to the Themes page to activate an installed theme.
To manually set an
active_theme you must define it directly in the next configuration file:
config/packages/liip_theme.yamlliip_theme: active_theme: 'acme-theme'