Run theme switching in mkdocs-material only on print event

[timvink]

I prefer dark mode in mkdocs-material, and going to the print site page, it will switch to light mode (for printing purposes). I do this using javascript.

It looks like you can run bits of javascript only when printing: https://stackoverflow.com/a/11060206/5525118

This enables switching the color theme from dark to white only at the moment is it needed, keeping the print page in the nice dark theme as well, and in effect hiding this color theme switch from users.

[timvink]

This will be part of the next release, unless it's incorporated into mkdocs-material itself in the meantime. (squidfunk/mkdocs-material#1973)