Documentation: Add a few details

[nilsrennebarth]

I tried to change bat's default theme to my liking (make header-rules easier to spot) and finally succeeded. Could you add the following items to the documentation? That will make things easier for people that want similar customizations.

• The file name for theme files in "$(bat --config-dir)/themes" must end with .tmTheme
• The default theme is either Monokai Extended or Monokai Extended Light, depending on the terminal being recognized as having a light or dark theme
• The name of a custom theme is taken from the file name, not from its name property.
• Most of the theme is for syntax coloring for various specific languages, but the settings.settings dict of the theme contains general settings for bat's own use. Among these is the key gutterForeground, which is not set in the default theme because it relies on the hardcoded default value.

Thanks for the great software.