Yep, the first style manager extension was released in 2005 (Stylish). Style manager extensions are a much more frictionless way to manage and share styles

The WebExtensions API has APIs for injecting styles: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web...

Stylish was also found to be malware. This is the problem with extensions over browser features. You just can’t trust them with such critical data.