The long answer is, if you're willing to disable systemwide security features, you can experiment with modifying the appearance of macOS, but to my knowledge no one has made a real attempt to in a while. Furthermore, the themes on display here fit a paradigm where the UI is a composite of bitmap images, whereas the modern macOS is largely built from vector graphics.
But if you vectorize every bitmap in an old theme, so that individual pixels of color are converted to vector graphic rectangles; if you learn how modern macOS builds its appearance; if you make a robust solution and thoroughly test it, so that it will work 100% of the time with every app ever; if you do _all_ of that, you will still be unable to _share_ your creation with most Mac users, because very few of us would disable systemwide security for the sake of running a third party system enhancement.
That's one of the major contributors to the success of Mac OS 9 theming: third party system extensions were commonplace, they were the backbone of the ecosystem, and Apple had no mechanism for preventing their use.