Function substitute_internal 

fn substitute_internal<'a>(
    variable_value: &'a VariableValue,
    custom_properties: &'a ComputedCustomProperties,
    stylist: &Stylist,
    computed_context: &Context<'_>,
) -> Result<Substitution<'a>, ()>

Replace var() and env() functions. Return Err(..) for invalid at computed time.