To clarify my question, a bit:
Given all this talk about 'Personalised Learning', is it possible for students to change the background colour, to something that makes the text more accessible for them? Specifically, I am thinking about dyslexic learners, where it has been identified that pale blue, or cream paper works best for them. I have tried the Accessibility Buttons on Moodle, and the Plugins on various Browsers, but the results are not quite right. They tends to knock out all existing background colours - which means the disadvantaged user is still missing out. It's a bit like wearing sunglasses, the sky might be less bright, but it is still blue.
Of course, such students could be supplied with transparent overlaps for their monitors. But somehow, this solution seems a bit primitive and pre-digital. And it wouldn't work so well, if they then moved devices, eg from wide-screen PC to hand-held iPad.
Although our Moodle theme has been designed to comply with all the DDA and Equal Opps stuff, it still relies on the assumption that 'one-size-fits-all', and anyone who is slightly different, be they dyslexic, a reluctant reader or 'just a luddite' is probably going to find the whole experience of Moodling, a little bit unpleasant.
I will have a go at creating 'clones' or multiple themes, for different people. But again, this solution takes the agency away from the user and implies in a traditional-teacher-voice, "I know what best". In a BYOD world, how far can Moodle accommodate different expectations?