Option to set theme mode independently of OS setting

[jvortmann]

I use the OS always on the dark setting but switch between dark and light themes in the terminal depending on the brightness of the day outside my window. I would like to use the existing theme setting to set a pair of themes, but set its mode separately.

Something like:

theme-mode: auto | dark | light # where 'auto' is the current behavior

This way I can programmatically change the mode and refresh the config (or eventually auto reload #3963).
Additionally a keybinding could change the mode (or the theme) after some API is available (#2353)