Check Indicator

The mark on a chosen option: a checkmark by default, and nothing at all when unchosen, so a listbox shows no empty box beside every row. This is the indicator to replace to change what "chosen" looks like: mapping it to RadioIndicator gives every single-selection mark radio visuals, including an empty circle on unchosen rows. Unlike the checkbox and radio visuals it renders no chrome of its own (it IS the glyph), so a host's theme target lands on the same element as astryx-icon.