A theme league is like a normal (or open) league except that it has special rules that all teams must follow. Generally the rules involve what players may be drafted and/or how players are used during the SimLeague season.
The person who creates a theme league determines the rules for that league. A few things (salary cap level, whether rookies are allowed, and whether the waiver wire is allowed) are set by the creator and enforced by WhatIfSports. But the creator is free to set any other rules he/she would like to use. For those rules, the league creator is responsible for enforcement (with the help of customer support).
Examples of popular theme leagues are:
- Franchise (where all players must be from the same franchise)
- Season (where all players must be from the same season)
- Draft (where owners pick players through a draft process and only one season of a player is allowed in the league)
- Blacklist (where specified players are not allowed to be in the league)