For some reason I have it in the back of my mind that they were at one point accused of being a honeypot for US intelligence because of their association with MIT. Probably complete BS, but maybe not. Are they as open source as they claim to be? Looks like they're on github. F-Droid seems to think they have some Google libraries or whatever that they use.
the "ads" aren't shoved up your ass. It's not bad.
their customer service helped me a couple of times. (And a couple of times they didn't)
they have a dedicated site for feature requests. Yet, they still choose what they implement and don't comunicate it perfectly. They're still a company. They do it better thab most others.
they don't artificially limit the features on ine os, because it's not yet implemented on all. That's actually a good thing, not a bad one.
on linux it's incredibly easy to add a vpn. No idea why we scream for an app.
As far as I can tell, the Linux desktop client doesn't have it yet, but has been promised.
EDIT: Ok, duh, on Linux Mint under System Settings>Themes>Settings>Miscellaneous Options, Dark Mode I selected "Prefer Dark" and voila: Dark. Proton's the only app I have that that setting seems to have any effect on . . .
It seems to be missing from the official app. But I believe it is an account-wide setting. So log into your web interface and change it from there and it should reflect on the app.