For digital painting I usually suggest people check out something like Krita or Affinity (they are already aware of Photoshop). I just don't have much firsthand experience with them since I felt like my needs were met with other software.
Sidenote: After actually downloading Krita it is more Photoshop-like than expected. I'll check it out as a replacement for myself. There's also been a huge, but very rare, need for Photoshop at my job (we use Linux) which has always been a problem (usually in modifying textures where Nuke isn't a good fit). The current plan was to try and find a CS6 license and either dual-boot Windows or use WINE or to buy Elements. Gimp was always too much to ask of them. I'll see if Krita fits that need.
Given that macOS has a reputation, deserved or not, of being the de facto OS for graphic designers, why do you think you have so relatively few Mac users?
This could be one of those places where the efficient market hypothesis is wrong because of incomplete information. The reputation of macOS for graphic designers is well known, but if the result of that is that everyone makes graphics design programs for macOS and not Linux then the macOS market gets saturated with supply and each supplier has to share the market with many competitors. Meanwhile the opposite on Linux, less competition so more volume per supplier.
It implies that Linux may be a good market for graphic design software because there is currently less supply there than demand. At least until more of the suppliers figure that out and release their software for it. (But that may itself cause more graphic designers to switch to Linux, which would then still justify the porting effort even as other suppliers move there too.)
For people who do it for their job, they're willing to invest in the hardware and software. I can't speak directly to painting software, but in my industry people really cling to the software they know or the rest of the industry uses. It's an uphill battle even if the software is better or cheaper to get adoption.
Let's compare comparable. Neither Cocoa nor Win32 do support third-party themes; for the exactly same reason why GTK is breaking them.
For OSX, there were also hacks that changed the system appearance, and Apple took increasingly harder stance against them. Again, they break more than they solve.
The reason why platform vendors do not support third-party theming is, that it limits their ability for further development. They would need to preserve compatibility for something, that is internal implementation detail. For first party themes, they can keep them in lockstep with the rest of framework. For third party, they cannot. So they prevent third parties from existing.
FOSS development can of course be funded via Patreon, donations, etc. But IMHO those models doesn’t seem to work so well for sophisticated GUI applications. (With some exceptions here and there.)
The mac ecosystem now has _multiple_ viable indesign and photoshop alternatives.
AppKit, Cocoa, UIKit and all those frameworks gradually added all the features third party app developers hacked into their own UI libraries.
GTK is something developers frequently curse about and theme developers frequently abandon because the gnome team keeps breaking their themes.
QT while great is nowhere near the ease of use of the cocoa frameworks and for every 20 cocoa books you find one QT book.
Bottom line is, as much as you may hate Xcode it's exponentially easier to make a good looking application on a mac compared to any linux counterpart. At the end of the day, people want to use (for example) photoshop or something similar without dual booting.