Function resolve_command_theme 

pub fn resolve_command_theme(
    arg: &str,
    registry: Option<&ThemeRegistry>,
) -> Option<String>

Resolve a /theme <name> argument to the spec apply_and_persist consumes. Built-in presets are case-folded; discovered custom themes keep the original case because the registry is case-sensitive and discovery preserves file stems (/theme Ocean must find Ocean.json). Returns None when the name matches neither a preset nor the registry.