Hypercard was absolutely dope as an entry-level programming environment.
Even modern desktop Linux pales in comparison because although it’s technically possible to change anything imaginable about it, to do a lot of things that extensions did you’re looking at at minimum writing your own DE/compositor/etc and at worst needing to tweak a whole stack of layers or wade through kernel code. Not really general user accessible.
Because extensions were capable of changing anything imaginable and often did so with tiny-niche tweaks and all targeted the same system, any moderately technically capable person could stack extensions (or conversely, disable system-provided ones which implemented a lot of stock functionality) and have a hyper-personalized system without ever writing a line of code or opening a terminal. It was beautiful, even if it was unstable.
You could also directly jump into the ExitToShell code in ROM (G 49F6D8, IIRC). Later versions of Minibug had an “es” command that more or less did the same thing (that direct jump always jumps into the ROM code, “es” would, I think, jump to any patched versions)
A point for discussion is whether image-based systems are the same kind of thing as OSes where system and applications are separate things, but if we include them, Smalltalk-80 is better in that regard. It doesn’t require you to reboot to install a new version of your patch (if you’re very careful, that’s sometimes possible in classic Mac OS, too, but it definitely is harder) and is/has an IDE that fully supports it.
Lisp systems and Self also have better support for it, I think.
of course modern smalltalks or st-inspired systems could still incorporate these ideas
Writing a MacOS classic extension wasn’t exactly easy. Debugging one could be a nightmare.
I’m not sure how GTK themes are done now, but they used to be very easy to make.
And it wasn’t just theming. Classic Mac OS extensions could do anything from add support for new hardware to overhaul the text rendering system entirely to giving dragged desktop icons gravity and inertia to adding a taskbar or a dock. The sky was the limit, and having a single common target to do any of those things (vs. being split between the kernel and a thousand layers/daemons/DEs/etc) meant that if it could be done, it probably had been.
