[idea] Dedicated background and border colours for terminal popups

[nikbrunner]

Idea / problem

Terminal popups currently use panel_bg only for their outer shell. Their PTY interior falls back to the normal terminal background, while the popup border uses global accent.

This makes popups blend into ordinary panes and couples their border styling to unrelated accent UI.

Requested change

Add two optional popup-specific theme tokens with these fallbacks:

popup_bg     → panel_bg
popup_border → accent

When unset, existing Themes remain compatible through the panel_bg and accent fallbacks.

This should apply to [[keys.command]] type = "popup" and equivalent plugin popup placements.

Illustrative configuration:

[theme.custom]
popup_bg = "#12352a"
popup_border = "#6fb59d"

Why

The fallback chain preserves the existing Theme model while allowing popups to form a distinct visual layer when a Theme needs it. It also separates popup borders from Herdr’s global navigation, pane-border, highlight, and selection accent.

If this direction fits, I’m happy to help test or prepare a focused PR after approval.