Issue description

I'd love to be able to configure prompt-cursor-colour by referencing some variable in my "theme" palette:

set -g prompt-cursor-colour '#{@yellow}'

That is currently invalid, and only something like that is accepted:

set -g prompt-cursor-colour '#f9e2af'

I realise that the other properties that I could configure using such variables were style strings outright, complete with possible boldness factor, foreground, background, et cet., while this one is a mere single colour.

Consider for instance the differences both in usage and implementation between prompt-cursor-colour and copy-mode-position-style:

set -g prompt-cursor-colour        '#f9e2af'
set -g copy-mode-position-style    'fg=#{@yellow} bg=#{@bg} bold'

Am I missing something and it currently is possible to work with variables for single colours like this?
Alternatively, would it make sense to bring that extra convenience to tmux?

Required information

Version: 3.7-next, built off d0caf0a3 (current master tip on GitHub today, 2026-03-07)