feat(pageColor): add option to invert pdf pageColor

[sirchnik]

For programmers or students working late at night, viewing white-background PDFs for specifications or instructions can strain the eyes. A dark background with white text is much easier on the eyes in such scenarios.

While pdf.js has native support for a high-contrast mode when enabled through force-colors css by the operating system, it also provides a configuration option called forcePageColors. This option is designed to enforce custom colors for the background (pageColorsBackground) and text (pageColorsForeground). Unfortunately, I haven’t been able to get this configuration to work as intended.

There’s some debate around the use of custom page colors. While someone has created a https://github.com/shivaprsd/doqment to address this issue, I believe it’s simpler to achieve the desired effect using pure CSS.

To that end, I’m using this CSS snippet: mozilla/pdf.js#2071 (comment). It inverts the colors of the PDF and works quite well for my needs.