Vim's automatic background color detection works in GUI mode, but not in a terminal.

On any xterm-compatible terminal, vim could detect the background color via terminal escapes (documented in http://invisible-island.net/xterm/ctlseqs/ctlseqs.html) and use that to automatically detect whether to use bg=dark or bg=light (by default or with :set bg&).

To detect the background color:

• Include `\e]11;?\a' during terminal initialization.
• Asynchronously, when processing input, watch for the escape sequence \e]11;COLOR\a (or equivalent using OSC instead of \e] for the escape, and ST or \e\\ instead of \a for the terminator). If the COLOR portion matches rgb:RRRR/GGGG/BBBB where R, G, and B are hex digits, then compute the luminance of the RGB color and classify it as light/dark accordingly.