Hi Jay,
The manual for the PRS-900 is here:
https://www.e-reading.club/readers/files/52/PRS900.pdf
You're right that there's no contrast adjustment on that model. There's also no newer software, either official or unofficial.
If you had a PRS-300/350/505/600/650/950, you could install PRS+ and its additional system settings which adds the contrast adjustment option.
Anyway, you can mitigate the issue a few ways:
1. use larger fonts
2. use the device in landscape for even larger fonts
3. edit the ePub to make all text bold,
see this thread
Good luck!