GH-139862: Fix direct instantiation of HelpFormatter

[savannahostrowski]

Based on #142274 (comment), the issue here is that format_help() eventually accesses self._theme but _theme was never set since _set_color() is no longer called in __init__. I think we can initialize _theme to a safe default (no color) so that direct instantiation works but the parser can still override it afterward.

• Issue: Investigate argparse formatter handling of color #139862

[savannahostrowski]

@hamdanal Mind also taking a look at this and making sure everything is fixed on your end?

[hamdanal]

This fixes the problem. Thank you.

[hugovk]

Worth adding a regression test?