This is the 465th most frequent Swedish word.
ovanligt
Unusual.
In this sentence, 'ovanligt' is used as an adverb modifying the adjective 'varmt' to express an atypical degree of warmth compared to what is expected for the season.
Det är ovanligt varmt för årstiden.
It is unusually warm for the season.
In this instance, 'ovanlig' (the base adjective form of 'ovanligt') is an adjective modifying the noun 'idé' to convey that the idea differs from the ordinary or expected types of ideas.
Jag tycker att hans idé är lite ovanlig.
I think his idea is a bit unusual.
Here, 'ovanligt' functions as an adjective describing the noun phrase 'Att se en räv i staden', indicating that such an occurrence is rare or uncommon.