change theme to the one, which has light/dark theme switch functionality

[pwittchen]

Examples:

• monochrome: https://kaiiiz.github.io/hugo-theme-monochrome/
• anubis: https://hugo-theme-anubis.netlify.app/
• minima: https://h.xjj.pub/ (I think, I like the most this one)

Or alternatively, implement light/dark theme switch button in book theme.

[pwittchen]

Another nice theme to try: papermod

[pwittchen]

Added papermod theme as default.