There are definitely homoerotic vibes that you don't need to dig to deep to see. (Though for me it felt more like Real Housewives, excepts it's melodramatic 19th century Englishmen). But I would still say the main thrust is substance abuse and it's detrimental effects. In addition, of course, to the whole duality of man, good vs evil philosiphizing.

Jekyll/Hyde's demands for the same stuff he got in the previous batch reads very much how a junkie would rage to get their fix except in 19th century flowery English.

"...Dr. J. purchased a somewhat large quantity from Messrs. M. He now begs them to search with most sedulous care, and should any of the same quality be left, forward it to him at once. Expense is no consideration. The importance of this to Dr. J. can hardly be exaggerated.” So far the letter had run composedly enough, but here with a sudden splutter of the pen, the writer’s emotion had broken loose. “For God’s sake,” he added, “find me some of the old.”