Thematic games are the games started with some opening moves already played. For example, French defence thematic starts with 1.e4 e6 already played, while Tarrasch (Prague variation) starts after 1. d4 d5 2. c4 e6 3. Nc3 c5 4. cxd5 exd5 5. Nf3 Nc6 6. g3 Nf6 7. Bg2 Be7 8. O-O O-O.
Thematics are frequently played by people wishing to explore some opening in depth or to learn and practice it.
2. Thematics on SchemingMind
Here on SchemingMind thematic games are available only in MiniTournaments.
The tournament organiser is able to setup the custom initial position for all games of given mini tournament. Most popular variations can be picked from dropdown on the mini-tournament creation screen, for even more one can use GameExplorer. Refer to MiniTournaments for more detail.
3. Thematics in individual games
While thematics are not available in individual games, there is a trivial workaround: one can use ConditionalMoves to offer the appropriate variation as initial moves of the game. Once accepted, the game proceeds as usual.
4. Handicap games
Handicap positions (like knight or pawn odds) are not available as such, but can be implemented by tricky knight variations. For example, stronger player playing white can offer 1.Nc3 Nf6 2.Ne4 Nd5 3.Ng3 Nf6 4.Nf5 Nd5 5.Nh6 Nf6 6.Ng8 Nxg8 as a conditional, obtaining the game with knight odds. Pawn, rook, or queen odds can be obtained similarly (white looses time by pointless knight moves while black uses knight to capture the appropriate piece.
See also this forum discussion.