On the contrary, I love marijuana themes in literature! (5)

'themes in literature' is the definition.
(plural of topos, a topic of theme)

'on the contrary i love marijuana' is the wordplay.
'on the contrary' says the letters should be written in reverse (letters in the opposite or contrary direction).
'i love' becomes 'io' (I've seen this in other clues).
'marijuana' becomes 'pot' (slang term for marijuana).
'io'+'pot'='iopot'
'iopot' reversed gives 'TOPOI'.