Thanks, Lorna, for reply and thanks, Michael, for further email correspondence. In case others have had the same problem, the solution lies in the 'Sharpness' control provided in my monitor, which, it turns out, is effectively a contrast control. The default setting for this is 50%, but I recently raised it to 100%. I have now found that reducing it to 70% to all intents and purposes removes the 'backlighting' effect in Flux.
Using the tests at http://www.lagom.nl/lcd-test/ confirms this. But it seems that 50% (the default setting) is the setting without artificial contrast-enhancement, and that is what I have now set my display to. With this setting, Flux is back to its excellent and comfortable self.