Function substitute 

pub fn substitute<'a>(
    variable_value: &'a VariableValue,
    substitution_functions: &'a ComputedSubstitutionFunctions,
    stylist: &Stylist,
    computed_context: &Context<'_>,
    attribute_tracker: &mut AttributeTracker<'_>,
) -> Result<SubstitutionResult<'a>, ()>

Replace var(), env(), and attr() functions, returning the resulting CSS string.