Function substitute 

pub fn substitute<'a>(
    variable_value: &'a VariableValue,
    custom_properties: &'a ComputedCustomProperties,
    stylist: &Stylist,
    computed_context: &Context<'_>,
    attr_provider: &dyn AttributeProvider,
) -> Result<Cow<'a, str>, ()>

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