My problems improved for a while, but have deteriorated again.
I'm always affected by an area of bright light (eg window & monitor) when everywhere else is darker and by variations in light levels. So no single solution works because I need to change settings when the light gets brighter or darker or when there's a bright light source.
I switched away from HC because there were too many exceptions that gave me problems and there were other solutions to explore. The HC extension in Chrome is helpful and settings can be switched instantly. I thoroughly explored available themes & colours in all the apps I used. I used WindowBlinds to put system stuff into a dark theme (for me, darkish colours are easier on the eye than black). I use colour filters on monitors and have a range of colour filtering glasses; filters can help, but also degrade acuity meaning there is always a tradeoff. I vary monitor brightness.
But this was never working properly and I have been constantly tweaking. It has really got in the way of getting back to my mini-review of writing apps.
On this I recently re-examined RightNote. Its ability to sync with other platforms & the web depends on Evernote. This only works with Evernote type pages. And these, it seems, can only show black text on a white background. Other page types are easily adjusted. I ought to report it, but, since none of the issues I raised to make it a more useful program for writing have appeared in any of updates, it didn't feel like a productive way forward.
So I've switched back to using global HC; basically thinking that it gives me the most stable foundation to work from. I don't know why images can't be excepted from its colour reversal. It's annoying for them all to be in the wrong colour and makes icons hard to recognise. I've increased the taskbar setting to help (I have always operated with small icons because my acuity is good). This solves the RightNote problem. Am now busy resetting all themes and default background and text colours. I still use the filters (sometimes on, sometimes off) and glasses (ditto) and vary monitor brightness.