Thanks for your reply.
> Maybe use the CO-1D Alt-Viewer configuration option to point to a app
> (console or GUI) that does the highlighting you need.
I could do that (or even use o or Alt-E to open an external editor), but that way I couldn't use F7 and shift-cursor keys, which is faster for comparing a couple of files. That's why I'm looking for an integrated solution.
> GUI solution
> Find or write a ZAAP
> I tried https://fossil.2of4.net/zaap/doc/trunk/ZPreview/readme.md,
> which used to work for me to view files but not today, I cannot remember
> if it does syntax highlighting.
I've used that many years ago for viewing graphic files. Just reinstalled it - for some file formats the file contents are being displayed, for others you'll just get a generic icon.
Seeing that you can use Total Commander lister plugins I installed a plugin called "Cudalister" - it sort of works, but also does not have Shift-cursor keys functionality.