There is always arc.
Theme is a product and reflection of arc. As characters change and arc away from their early positions, so theme will manifest itself through that.
Therefore there will always be a theme.
Writers may start thinking about a goal (get the golden fleece) and how that goal may be achieved. During that process they'll involve themselves with change/arc and the theme will be a reflection or product of that.
Else writers may be more direct. They'll think of a theme, decide how the characters change/arc to show that and then layer a golden fleece on top as the driver to achieve the initial goal.
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