Detect the terminal background (via COLORFGBG or an OSC 11 query) before
GUI init and, on the first start, automatically apply the built-in
"light" theme when a light terminal is detected.
The function gui_term_theme_is_light returns an int (1 if light, 0 otherwise,
0 being the safe value when detection is unsure).
Sébastien Helleu