cli (base): better background detection: - ANSI escape code detection is only used on terminals known to be compatible (currently xterm, konsole, and all VTE based ones) - `COLORFGBG` environment variable is used as fallback - if both methods fail, `dark` is used This should avoid problems with incompatible terminals (e.g. on QTerminal, users had to press `[ENTER]` each time). fix 406

You'll find install instructions in documentation (see doc/installation.rst).

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