Per-theme-mode profile icons

# Description of the new feature/enhancement

It would be great to be able to set icons per theme-mode (ie dark/light) - as it stands now a monochromatic icon can look very out of place in the opposite mode. Or maybe this is already possible somehow?

# Proposed technical implementation details (optional)

Allow setting dark/light profile icons individually in `settings.json`

Start by reviewing the profile icon and theme-mode settings described in settings.json, then trace where those settings are handled. Done means users can configure separate dark and light profile icons and each icon appears in the corresponding theme mode.

Written by the indexing model from the issue text.