The question is: can we design deep configurability (and composeability), and hence put more accessible creative power in the hands of the user (instead of say having to edit obscure configuration files or even worse, do a full recompile, or simply have no choice at all), without exploding complexity of the system or is this fundamentally mutually exclusive?
Power + complexity will only ever remain niche. But if we can design a system (be it a single program or an entire machine with OS), that is both powerful but intuitive to use, even down to deep layers, then we could actually empower 'end' users into becoming authors of 'creative computing'.
I don't think that's the concern that led to the loss of color configuration. You probably aren't hardcoding colors anyway, but using widget toolkits and the like. It's mostly because marketers wanted it to look pretty, and that means more complex widgets, so they can't be configured as easily as the relatively flat UIs of Windows 2000 and before.
(The new era of flat uncustomisable flat widgets is a direct continuation of the branding approach — its their marketers saying "I know better than you what your working tools should look like".)
This attitude of “we know better than the user what the user wants” is pervasive throughout technology, not just software. Products of all kinds are getting less configurable, less adaptable for different purposes, less integratable with other products, and less suitable for uses beyond what the manufacturer intends.
If hammers were invented today, they would be locked to a particular manufacturer’s nails, have software that prevented them from hitting anything else besides nails, and have DRM that self-destructs the tool if you stopped paying the $5/mon subscription.
Something like X resources gave the user deep configurability indeed; what's more, because they lived on the X server they communicated user preference to even remote clients.
A graphical tool to allow users to set preferences through X resources, similar perhaps to Visual Studio Code's preferences pane, might be a great way to allow users to tweak their apps with power and precision.
Of course, literally no one uses X resources these days, all settings live in XDG_CONFIG_HOME. And it's all moot under Wayland, which doesn't even allow remote clients.
Power + complexity will only ever remain niche. But if we can design a system (be it a single program or an entire machine with OS), that is both powerful but intuitive to use, even down to deep layers, then we could actually empower 'end' users into becoming authors of 'creative computing'.