Whether you want to further customize a style or create a new style, arranger keyboards have the functionality to perform the job. BUT...For whatever reason, each of the "Big 3" have gone out of their way to make programming/creating a style a little more difficult than it should be IMHO.
I must agree that making styles from scratch, as shown by Bert, will be beyond the skills (or interest) of many people, and also, some of us get lazy when there are lots of styles available for purchase or sharing.
At my follow-up clinics, I have managed to teach quite a few people how to use Style Assembly, which is probably the easiest way to make a style your own...some stayed with it, and others tended to drift away and become complacent and forgot how.
But it is teachable and doable.
Many of us who have worked with multi-tracking, be it via tape recorders or the later digital sequencers, can manage a bit better with putting in parts, but usually there's a section that might give us trouble, be it bass lines, drums, strums, piano parts, and this is where the swapping of parts is very helpful.
Stylemakers for the Big 3 use special sequencing programs and download..they never program on the instrument...I doubt if these programs will be made available to the public...I have actually asked about them several years ago, but was told they were not available. Of course, some people buy whole new arrangers just to get more variations of styles they already have...so I suppose style making on the instrument could be just hard enough so that only a few can manage, or bother, with them, as arrangers must be sold to keep the profits up, the company stays in business to make more new ones, and the beat goes on.
I think I've managed to get out of the loop with my present arranger, a Tyros4...and I'm planning to stay out.

I actually enjoy making my own styles, or should that be,
making styles my own?

Ian