Involved in the biosynthesis of theobromine (PubMed:16333668). Catalyzes the conversion of 7-methylxanthine (7mX) to theobromine but not able to convert paraxanthine to caffeine (PubMed:16333668). 7-methylxanthine + S-adenosyl-L-methionine = H(+) + S-adenosyl-L-homocysteine + theobromine Binds 1 Mg(2+) ion per subunit. Optimum pH is 9.0-9.5. Alkaloid biosynthesis. Belongs to the methyltransferase superfamily. Type-7 methyltransferase family.