There is now a hack to add a user-accessible userStyle.css on the PRS-505, see
Replace DEFAULT epub fonts! (PRS-505). So it is the first EInk device to allow this. This was very easy implement, pepak replaced 9 characters in the executable image! If Jinke (rather than a user hack) implemented this they would change the source code, but we now know a simple approach is to change adobe/pfx to abook/css (say) in the actual binary executable image (with this approach we must change 9 characters to another 9 characters).