CLI colors are hard to read on light-background terminals

A user on a light-background terminal reported that the miren CLI's color theme is hard to read. The CLI leans on bright colors (yellow section headers, yellow table column headings, green accents) that were tuned for dark terminals. On a light background several of these wash out — pale yellow on white is the worst offender, and the muted gray secondary text goes low-contrast too.

The fix we want is background detection: sniff the terminal's background (light vs dark) and adapt the palette accordingly, rather than shipping a single dark-tuned theme. Worth respecting NO_COLOR and any standard theme hints while we're in there, and a manual override for cases where detection guesses wrong.

Reported by a community member (peetzweg).