How to turn an Ubuntu Touch device into the new Suru Dark instead of the default Ambiance theme to have a black and energy saving user interface?
First of all a security note: Executing the following steps is at your own risk. I tested these steps only once at my BQ Aquaris E4.5 phone with an installed OTA10 update. I don't assume liability for this. Some native apps (like Browser or Dekko) don't support the new theme completely. You can see this in still white backgrounds or inappropriate font colors. But the functionality of these apps is not restricted.
So if you thought about it deeply, we now can start:
You'll need the Terminal app from the Ubuntu store. Open it and type the line:
After this press enter and change “Ambiance” to “SuruDark” (without the quotes and not with a blank, so not “Suru Dark”) at the end of the “theme=” line. Then press Ctrl-O in the Nano section of the terminal to save it, then enter and finally Ctrl-X.
To test your changes reboot the device. If you're not happy with the Suru Dark theme change “SuruDark” back to “Ambiance” in the similar manner like above.