Function compile_rule_styled 

pub fn compile_rule_styled(
    pattern: &str,
    hide: &[u32],
    slot: Option<&str>,
    theme: Option<&dyn ThemeRegistry>,
) -> Result<ConcealRule, ConcealRuleError>

OL.1: compile_rule, plus the style its visible remainder takes.

slot is a capture or theme-element NAME, resolved here — at compile time, once, the way query sources are — rather than per line. It goes through name_to_style_with_theme, which is the same path a highlights.scm capture takes, so a concealed link is coloured by the same vocabulary and the same active colourscheme as everything else. An unresolvable name is Style::Default: the rule still conceals, it just does not paint, which is the right severity for a theme that has not registered an element yet.