I've had a couple of discussions with the team.
@majorincident - first thing I need to say here is that the browser you are using is literally the oldest version of Firefox that giffgaff can officially support. If you are experiencing any problems with the community without any modifications (such as the one mentioned in this thread), then please do let us know and we'll look to see if we can fix those.
Looking specifically at your issue with not being able to use the browser extension discussed in this thread: unfortunately that is not something we will be putting any time or effort into. Compatibility of third party tools is not something that we are responsible for and we have far more important work to be progressing with.
Having said that, we are already looking at the feasibility of having a "dark mode" officially supported in the future. It isn't something likely to be made available for a while, however, as we do have other more important work to carry on with.