If there are going to be alternative themes, the first job is for them to work! I use Ambiant Dark, but it gets applied patchily. Even default apps, like Firefox, are not rendered properly. It is pathetic, truly. I find myself often squinting to read light grey text on a white background, or black text on a dark grey background, and such utter nonsense.
This is probably the clearest demonstration of why the Linux desktop is going nowhere. After more than two decades it still cannot implement desktop themes properly, such that they get applied uniformly across all apps, or even the default apps.
@Isaiah_Sellassie - could you please post the links to the bugs you've filed, with screenshots, so that us volunteer developers are aware of them and can address this "pathetic" situation?
Firefox is a bad example, since that actually uses their own custom GTK build and is almost guaranteed to not work properly as a result: the scrollbars, for example, have been wrong for far more YEARS than they've been right.
Since Firefox is also one of very few apps that also lets the CONTENT choose the color scheme, you're even further off track there. There are a LOT of websites built by people with no technical ability at all, and if those say "Black Text" or "White Background" WITHOUT the other half - and a huge number of them do - then you're going to end up with the outcome you describe when you change your colors, especially if you don't FORCE those colors in Firefox's settings.
I think you're misunderstanding what "default apps" means though. It DOESN'T mean "made by the MATE team", or even by Canonical. It simply means "what's pre-installed", and those default apps are predominantly from unrelated developers / companies.
Yes, I understand that distros are patchwork quilts. Every project has its own interests and priorities. Developers are doing what they enjoy doing, or solving some specific problem. I understand Firefox is its own project.
Either Firefox should not be the default browser, or there should not be alternative themes offered. If you care about the user experience, you have to choose one or the other. If I were making the decision, I would eliminate alternative themes until a standard is achieved that all projects are coding the app UI to. This is easy to do for a single company, but probably impossible for an open source project.
I have filed 5 or 6 bug reports before, around the time of the 18.04 release. None of the problems has been fixed. I am not so invested any more to devote time for bug reporting. When the system prompts me to send error reports, I always do so.
The specific problem of themes is not a bug, in my opinion. It is an architectural sort of issue. It isn't one isolated app rendering one isolated element wrongly. It is a problem that occurs across many apps, e.g. Kdenlive, off the top of my head.
I am making a general point, and not directing it to the Mate team as such.