Function resolve_theme 

pub fn resolve_theme(
    platform_id: &str,
    content: &Content,
    global_config: &Config,
    profile_default_theme: Option<&str>,
    registry: &ThemeRegistry,
    fallback: &Theme,
) -> Theme

Resolve theme using 5-level resolution chain (similar to RFC-0005’s published resolution):

1. meta.toml[platforms.X].theme — per-content platform-specific
2. meta.toml.theme — per-content default
3. typub.toml[platforms.X].theme — global platform-specific
4. typub.toml.theme — global default
5. Profile default_theme — hardcoded in profiles.toml

Falls back to the provided fallback theme if no match at any layer.