expectation failed

With PC GUI: APKToolGUI

With Android APK: APKExtractor

Terminal: combinating jar files like APKEditor.jar, apktool.jar, apksigner.jar

Bonus points if Anyone can link me to the page I used a few years back that had all sorts of privacy enhancing and telemetry disabling option on the left side and would create a reg file for applying those changes on the right

Sounds like https://privacy.sexy/ might be the answer?

Are you referring to generating CSS code or their own CSS (design) on the website? For the second, you can try it with the GPThemes browser extension

FlorisBoard with Material You dynamic themes version