I think
@Umbran is saying that the decisions are kinda analogous. I think I more or less agree:
When one is choosing what one's character will be, one can choose starting from the theme--and then find mechanics to suit--or one can find mechanics that seem attractive for whatever reason--and then find a theme that meshes.
In a similar way, when one is looking at possible campaign ideas, one can look at possible campaign themes, then pair those up with game systems that support them; or one can start from a game system and then look for themes that seem consistent with its mechanics.