I understand that this may feel limiting or weird, and in some instances probably unacceptable. I have a hard time imagining Facebook making a native app that doesn’t 100% reflect the Facebook brand. (I also have a hard time imagining Facebook making a native app.)
On the other hand, look at Tooter, the Mastodon client. It doesn’t show off any specific branding, neither app brand nor instance brand, but does that actually seem weird to you, as a user?
In fact, I feel the design system is elegant in practice, not just in principle. Let me explain why.
If you look at SailfishOS as a whole, there is very little branding going on. Instead, things are centered around user wishes. Have a favourite colour? Pick an Ambiance image and colours to match your preference1 - and the system will adapt and still feel consistent.
Same goes for apps. First and foremost, these are tools for people and canvases for content. I mean, look at how screen real estate is allocated. It’s almost 100% devoted to the content a person is interested in, the very reason they run an app.
There’s no ever-present navigation bar or worse, one that slides in and out by itself. There’s not even a strip of indicators telling you battery level or signal strength. Contrast this with Android or Phosh, for example. (It’s always just a quick peek away, for when you really want to check.)
First, a quick note on consistency. Apps can have their unique visual identity without breaking consistency only if the underlying system is already a mess. I.e, when there is not much consistency to break.
In general I think there are two specific places for apps (and app developers) to brand themselves: The icon and the cover. In both cases branding helps people quickly distinguish apps, in addition to providing brand recognition.
Other than that, the most important qualities of an app are how well it serves its purpose, and how native it feels. People shouldn’t have to learn unique visual cues or procedures to accomplish what they want.
Said in another way, apps and the underlying system are guests in people’s homes. They should follow the whims and rules of the host. This is different from people visiting a shop downtown, or a website, in which case the owner gets to decide - within the rules of the law (unless the owner is too big to care).
I understand the wish for a visual identity of one’s own and I do think @nephros’s example above is reasonable branding. There may be cases where the aesthetic or mood is the reason a person starts an app, and then it’s entirely reasonable that the app provides exactly that.
But please also consider the cases where a person has made their opinion known by picking a suitable Ambiance for this specific moment.
Continuing the home metaphor: As a guest, how much branding can you add to their house without being offending? Will your vivid colours and fancy fonts be well received, or will you look like an intruding clown because they chose an Ambiance conveying tranquility?
Other platforms say “stop theming our apps”. People using SailfishOS are told to “theme the system and apps as you wish, and change theme as often as you please”.
(Edit to make footnote visible:)
1Pick <…> colours to match your preference and sound settings to go with them. Ambiances are such an underestimated vehicle for setting your phone up for any occasion, especially when combined with the brilliant Situations app.