There are a number of themes which emerge in "The Red-Headed League." One of the most striking of these themes is best summarised as the bizarre and unusual in everyday life. This theme appears at many instances in the story and represents a shared interest of both Holmes and Watson, as Holmes comments:
Now, my dear Watson, that you share my love of all that is bizarre and outside the conventions and humdrum routine of everyday life.
This theme is also represented by the Red-Headed League itself, a very unusual group whose membership is based exclusively on having fiery red hair. The bizarre also features in Jabez Wilson's work for the League: he is tasked with the unusual job of copying out pages of an encyclopedia.
Greed represents another important theme in "The Red-Headed League." As mentioned in the reference link provided, Wilson's greed provides the impetus for the story, as saving money is his only motivation for hiring Vincent Spaulding (also known as John Clay) to work in his pawn shop. Conversely, for Spaulding, greed is the driving force in his decision to invent the Red-Headed League and, in the longer term, to steal French gold from the City and Suburban Bank. Greed, then, provides the story's conflict and dramatic climax.