I have had to install a (Windows) PC for somebody who needed low contrast themes in a previous job. This woman could only see "light" and no colours, and had low vision in general. (I think she said she had no cone cells or something like that, but it's been 2 years). She used a low contrast theme in Windows that used 2 shades of blue.
She described looking at a screen with high contrast (e.g. black on white or white on black text) as "looking straight into a spotlight and trying to see/read something that's written on it".
At least one other almost blind person told me it was the same or similar for him (but his eyesight is so bad now that he does everything with a braille reader & TTS anyway—he also uses the console most of the time).
BTW: I think it's possible to use compiz filters to show "low contrast" & "high contrast" versions of a normal display, but I don't know how well that works in practice?