Outline X for Substratum (Extensions) is a personalization app designed for Android. It is an extension of the Outline and Ethereal themes, offering additional colors and tweaks. The app is compatible with Nougat, Oreo, and Pie versions of Android. The app offers accent colored icons in settings, filled and invisible softkeys, and an alternative brightness icon in QuickSettings. Additionally, Nougat users can customize their QuickSettings background colors, while Oreousers can enjoy a rounded searchbar in settings. The app is available for free and can be downloaded by following the HowTo guide, which involves installing Outline/Ethereal Theme and then installing the Tweaks. Users can suggest additional colors and tweaks in the developer's Google+ Community or add them on the Extension's Github.