You can copy in the good ones from the LD10 theme list or add new themes! As a general rule, try to avoid themes that either require specific cultural knowledge (including sports), or impose implementation limitations (like “2-bit color”).
(let's not use these again for a while)
LD10.5: Weird/Unexpected/Surprise
LD10: Chain Reaction
LD9: Build the level you play
LD8.5: Moon, Anti-Text
LD8: Swarms
LD7: Growth
LD6: Light and Darkness
LD5: Random
LD4: Infection
LD3: Preparation
LD2: Construction/Destruction (Minor Theme: Sheep)
LD1: Guardian
LD0: Indirect Interaction
72HGDC #8 Nov '06: Magnets
72HGDC #7 Jul '06: Retro
72HGDC #6 Mar '06: Magic
72HGDC #5 Jul '05: Horror
72HGDC #4 Dec '04: Swappable Parts/Customize?
72HGDC #3 Jun '04: Pirates
72HGDC #2 Jan '04: Food
72HGDC #1 Jul '03: Mindless violence
FGDC4: Ball Physics
FGDC3: Replay
FGDC2: Grow
FGDC1: Point & Click