Did you actally read the thread? His rage-quit feels actually quite tragic right now. Additionally to the insane problems with Google and the PlayStore-enviroment/ecosystem, his girlfriend is currently sick, and taking care of her is his priority.
I'd find it more appropriate if the (current)top-comment doesn't casually criticize some simple looks of the app as if nothing. I mean if somebody's drowning right now next to me, I don't want to hear commentary about how shitty her/his shoes look like. Well, you didn't choose to be top-comment right now for everybody to see this like on a silver plate, but to everybody else... In real life I'd suggest: maybe discuss this on some other day.
I like the UI, it works well is easy to configure and gives me everything I need. I tend to dislike new UIs that have hidden defaults, are not configurable enough (because users won't understand) and focus on form over function. FairEmail is great because it's super useable and gets out of my way when I want to do anything.
Props for keeping up the apk though. For everyone using the app I hope it'll live on as fork.