Is there any way for me as a reader of some interactive fiction games, to edit page styles/colour schemes?  I have asked the author but they haven't replied - and I am struggling as the colour scheme is really glaring.  

I've looked for accessibility options; tried a 'dark mode' setting option in the game (did nothing); blocked the wallpapers -  but in terms of formal settings can't see anything where I may be able to force something a bit more...readable.

That depends on what engine the game is made with, and where it's running (in the browser, or something like Gargoyle?) But even if workarounds are possible, it's mainly up to the game creator to consider accessibility.

If the game is actually HTML content, you can always force a style in your browser.

Look for tutorials on local overriding or local CSS diff, or you can look for a more user-friendly extension that lets you modify styles locally.