If a game is abstract, the rules should be brief. Rules can only be substantial if they encompass a theme.
Quotes about theme
The mechanics are the abstract means by which the game works. The metaphor is the game’s beautiful lie, the fiction that gives the game context and a broader meaning.
People think my games are not thematic because I don’t have a different rule for every piece.
All told, if a designer has to pick one or the other, more games can find success with a weak theme and a strong mechanical core. Whereas games with an incredibly cool theme but weak or boring mechanics very rarely stick around for long.