Thanks.
I ended up selecting MacDown’s Github 2.0 CSS (under ~/Library/Application Support/MacDown/Styles if you have it installed) in DEVONthink’s preferences and then adding and linking the Prism files as required, seems to work OK.
So the main issue still, as mentioned above and in this post from a year ago, is the preview window not updating as a user types.
Looks like DEVONthink has some really powerful features but update cycles and apps generally looking dated has definitely been a concern while assessing and a concern expressed by numerous others, including some prominent reviews, apparently over many years. Not sure why this hasn’t been addressed?