Though in the past, Tokuma Shoten was associated with a number of entertainment industries, in modern times it is primarily associated with publishing. Its role in the founding of Studio Ghibli came about due to the success of the Nausicaä film which was an adaptation of the manga, Kaze no Tani no Nausicaä, which was being published by Tokuma Shoten in their Animage magazine.
However, that magazine mostly retained a strong focus on animation over manga and though Tokuma Shoten has had some significant manga magazines in decades past such as Monthly Shonen Captain (1985-1997), today it's only ongoing manga magazines from before 2006 are Chara and Chara Selection which began in the 90's and are aimed at the niche boys' love market.
For mainstream manga, their only current manga magazines are two seinen monthlies, Comic Ryu (launched in 2006) and Comic Zenon (launched in 2010). Comic Zenon is notable for coming about rather fully formed due to it being part of a partnership between Tokuma Shoten and the company Coamix who had parted ways with Shinchosha where they had created the Weekly Comic Bunch magazine. As such, Zenon continued a number of series from Comic Bunch and has serialized numerous spin-off's to the Shonen Jump works of Tsukasa Hojo and Tetsuo Hara (two of the founders of Coamix).