A useful distinction
A matching game can use fruit, stars or abstract shapes while keeping the same matching rule. Conversely, two games with similar artwork can demand different decisions. Separating theme from mechanic makes early design discussions more precise.
Put it into practice
Replace the pictures in a paper prototype with labelled shapes. Check whether the rules still make sense. Then note which information the original artwork supplied, such as category or orientation, so a visual change does not accidentally remove a cue the player needs.