Pdf.js: Photon design

[nt1m]

It would be nice if PDF.js's design would be in line with Firefox's.

To do that, here are some quick changes that would be possible:

• Remove background gradients/textures
• Use photon colors: https://design.firefox.com/photon/visuals/color.html#grey (use visual layering to pick shades of gray: https://mozilla.invisionapp.com/share/J6GQCZR2PZY#/screens/289803032_Desktop_Common)
• Use photon icons: https://design.firefox.com/icons

[Huluti]

It's true that the reader looks very outdated... compared to the browser

[timvandermeij]

Fixed by #11077.