I could also add a theme for colour blindness where reds and blues are seen as only shades of greys.
Not sure that would help - but it's an interesting thought. I suspect the variety in colour blindness effects would mean you would need to allow lots of user choices for anything that displays different colours for text, backgrounds and borders (so that people could find a combination that "works" for them), rather than reducing everything to greys.
n.b. Colour blindness is a "cones" issue. The contrast sensitivity, due to ageing, is a "rods" issue.
It seems the number of rods to cones, in the central portion of the retina, declines as we age. The rods work in monochrome, discern levels of contrast, and work well at low light levels. The cones discern colour differences - but need a decent level of light to work.
Actually, people with some forms of colour blindness do have a lot more rods than cones in the centre of their retina - so they can discern lots of shades, even in low light. Your "dark theme" should already be a help for these people, as they can find bright displays very tiring.