Allow dark/light theme switch on user browser

[SijieFu]

1. New fearture: dark/light theme toggle on the webpage with user preference. Solves toggle between dark and light mode #911.
   • html[data-theme="dark"] selector to overwrite default light theme variables.
   • localStorage to store user preference of the theme. This is for theme consistency. User preference overwrites the default theme at compile.
2. Fix: Navigation text overflow and wide empty spaces on mobile screens (or width-limited screens).
   • Deprecated $masthead-height to position the sidebar and the main body below the navbar. When the site title is long, this causes overflow to the fixed height. $masthead-height definition is still kept as a backup.
   • The navbar height is set to fit-content and the padding-top for the sidebar and the main body is updated together with updateNav() function.
   • Minor updates to related padding/margin to fix large empty spaces.

[rjzupkoii]

Thanks for the pull request and hard work to get this running!