I maybe wrong here, but I think 'inverted ambience' means this; white wallpaper with black text/icons. I have already created a white ambience, but it doesn't look too great. The Settings menu is very grey in the background with thin black text on it. The homescreen, locked or not, has a mixture of black and white icons, for example; The battery percentage text is black but the battery image is still white, same with internet activity icon, it's black, but network reception remains white and of course, with a white background, white icons are just not very visible.

Don't wait for Jolla to develop this new idea at any great speed, it's been 4 years since we've seen any actual, tangible updates and sadly, the ambience module is proprietary/not OSS, so we can't (well, I can't) do anything with it ourselves.

I noticed that Russian Sailfish has capability for inverted ambience, but I have no idea if it is fully implemented yet, I'm certain I have seen it in an image somewhere, possibly a link from Coderus signature here on TMO, but of course, so many images these days are just CGI mock-ups and have yet to materialise.