Perhaps one of CyanogenMod's greatest features was its built-in theme engine. It wasn't perfect, but it got the job done and gave users a unique look. Now that CyangenMod is defunct, and the developers behind its successor, LineageOS, have confirmed they won't be continuing the theme engine, users have to find another way to theme their devices. No need to fear, though, Substratum is here.
Substratum theme engine is an active theming solution being developed by the very same team that brought you many popular CM13 themes. It's fully functional on LineageOS, as well as almost any AOSP-based custom ROM, plus it can even theme the stock firmware on devices that use a vanilla Android interface. As long as you're rooted, installation is incredibly simple, so I'll show you how it works below.
Begin by installing "substratum theme engine" from the Google Play Store, which you can find at the link below. Then, launch the app and quickly go through the introduction. I usually skip past the introduction, but in this case, you'll learn a bit about how the app works, so it might be worth looking at.
Once you're done with the introduction, tap "Allow" when prompted to grant the app permissions, then press "Grant" on the Superuser request popup. The app will explain what the permissions are for, and root is required to ensure the theming aspect works as intended.
To demonstrate how theming works, we'll be installing Default Dark Substratum Theme, but you can install any substratum theme that catches your eye. Searching for "substratum themes" on the Play Store will give you plenty of results to pick from. Once you've found one you like, just install it like any other app.
After installing a theme, go back to the substratum app and head to the Theme Packs page, then select the theme you just installed. If you'd like to apply the entire theme, enable the switch next to "Select to toggle all overlays." Otherwise, you can pick and choose the individual apps and elements that you'd like to theme by ticking the boxes next to them.
When you're done, hit the floating action button and tap "Install Selected." Give your phone a minute to compile and install all of the overlays. You'll see a progress bar — wait for that to go away. As soon as everything has finished, go ahead and reboot your phone to allow the theming engine to apply the overlays.
After your phone finishes rebooting, you'll notice that the selected overlays have been applied. Depending on the theme you chose, various aspects of your phone will now have a fresh new look. Besides stock applications like SystemUI, most developers also include themes for many popular third-party applications such as WhatsApp and Skype.
While CyanogenMod's theme engine will surely be missed, substratum has proven it's more than capable of filling in the gap left behind in LineageOS. There might not be as many themes available at the moment, but there are plenty of developers in the process of porting their themes over. In the meantime, be sure to leave a comment down below letting us how much of a difference substratum has made on your Android phone!