To me, when I think “abstract”, I think “no theme”. Games that aren’t abstract have a theme that helps make sense of the rules of the game.

There are also games with themes that don’t really help make sense of the rules — the theme is just pasted on. These are still considered abstract, despite technically having a theme.

The “ameritrash” genre are known for having strong themes that tie to the rules of the game. E.g. a lot of co-op games with plastic figures.

Old school euro games often have a pasted on theme, and are more abstract.