Fish 4.3 Update on universal variable

[theopn]

They are not yet phasing out universal variables (fish-shell/fish-shell#8455), but in 4.3, the theme mechanism transitioned away from using universal variables (fish-shell/fish-shell@190d367).
Add theme event handler in the config, and consider potentially not relying on universal variables.

See also:

• Remove default universal variables fish-shell/fish-shell#7317
• Remove universal variables fish-shell/fish-shell#7379