German board games have a self-rated "suggested age" that is more about the cognitive difficulty and the themes, so a non-violent game about politics would be 12+, Chutes & Ladders is 4+ and Catan is 10+. Probably not a lot of four-year-olds on itch though...
But in that context, "adult themes" would be more along the lines of "Kirby does his taxes".