On universality:

A good game is a game which is loved by its players, but IMO it doesn't really matter for the quality of the game if this set of players is large or small. (It is a good product if moreover it's player base is large.)

As a consequence, a game should either be universal enough so that every players at tables picking it find something to love, or be advertised in such a way that only tables that will love it start a campaign (or a balance of the two).

On theme vs quality of the game:

Every RPG or boardgame has an underlying "abstract game" where you remove all its theme.
+ This abstract gameplay can be good or bad by itself.
+ The theme can serve the gameplay by having the intuition of the players help them to remember the rules and their interactions (monsters are powerful, insects are individually weak, etc).
+ At the contrary, the theme can disturb the gameplay by having "optimal strategies" look stupid and unintuitive (e.g., the best way to run your ancient tribe it to let your peoples starve and never produce any food, because there is no actual death in the game, just negative points for not feeding them)
+ In the case of a RPG, the theme is both the most important and the least important part. It is the most important part because RPGs is at is core a story building game. It is the least important because whatever is written in the book will often be overshadowed by whatever the players and GM bring to the table.