In looking for hymns with the topic "Advent Candles" or "Advent Candle lighting" I was surprised not to find "One Candle Is Lit" listed. I think such a tag is obviously in order, but I don't see how to add it to the text authority.
Topics headings come directly from the hymnals so they are specific to the instances in the hymnals. They cannot be added to an authority. Some hymnals use the topic "Advent Candle Lighting" but the Chalice Hymnal does not; it uses "Advent" and "Light." Topics should not be added to an instance unless the topic is included in the hymnal's topic index or the hymnals sections. Some day we will probably map topics so that a search for something like "Coming of Christ" would map to Advent.
You can try searching for "candle" in the text name or full text fields and "Advent" in the topic field to find hymns about lighting advent candles.