Settings are located at Edit / Project Settings… / UI Themes. If you using New UI Widgets then settings are shared New UI Widgets and located at Edit / Project Settings… / New UI Widgets
Enable/disable assembly definitions. Enabled by default.
Enable/disable TextMeshPro Support. Enabled by default if the TextMeshPro is installed.
Support is enabled only to installed platforms. Platforms that were added after it requires enabling support again.