Despite the embarrassment and "Doh!" moment..
So, one screen has RLM with a pretty-in-black site layout. The other screen has a Firefox preview of the page, ugly in mainly default colours.
I spent a while hunting forums and so on wondering why text and background colours weren't coming through like they should.
I'd started on the post title, noticed I didn't have an open preview any more and wanted a screenshot of the two views side-by-side, but accidentally clicked the "preview" button rather than "preview on". 'Twas only when I did that that I saw a hidden breakpoint which would be under "Layout" on the tool bar.
Get rid of breakpoint, all comes back to as it should.
(Request for future development - to be able to carry style over to the RH side of a breakpoint as well as LH (or for someone to tell me how to do this so I don't have to re-do all the colour stuff again )