Você pode tentar modificar a origem desse fork de MuPDF , que é ParaPDF :
paraPDF is a fork of MuPDF. It's simply intended to add a few features muPDF was missing which I was interested in, if I can't get them to upstream, and add some additional keybindings to make it feel more like home (home being vim).
Specifically, here's the changes I'm workig on:
Reload the PDF when receiving a SIGHUP. This can be useful, for example, when working with LaTeX or something else that outputs PDFs.
Create a keybinding to invert the colors. This can be useful if one uses a dark color scheme. It would be preferable to refrain from inverting images, but that's likely to much work for the effort (and my capabilities).
Added a flag to start with colors inverted.
Add some bindings to make muPDF feel a bit more like vim. Most notably, ctrl-e, ctrl-y, ctrl-f, ctrl-b, and gg.
muPDF's original README follows (with my addition to the copyright):
Caso contrário, você pode converter o plano de fundo branco do seu PDF para qualquer cor desejada, seguindo o procedimento que descrevi aqui . Trabalhe em uma cópia do PDF e use esse PDF resultante quando quiser lê-lo com um plano de fundo cinza. Isso só funciona para PDFs com um fundo de vetor.
No seu caso, você muda para cinza, mas a lógica é a mesma para qualquer outra cor que você queira definir.