Admin theme editor not loading

[BastianHammer]

Either via an extension, yes.
Or, there have been some threads around custom CSS and custom JS, how to include those and possible workarounds like:

When will be able to make new themes for 8.2?

What’s the roadmap looking for opening up the ability to modify and customize themes? Quick tip: Chrome & Brave users can install “USER CSS” extension then modify the color’s CSS in Inspector and copy and paste the modify CSS into the extension for your CRM instance.