(Black Butler, dark-themed, really?)
Well there's Tokyo Ghoul:re (I'm just trolling, you've probably read it already)
http://www.mangago.zone/read-manga/alive_the_final_evolution/
http://www.mangago.zone/read-manga/ares/ starts light then gives you ptsd
http://www.mangago.zone/read-manga/berserk/ obviously
http://www.mangago.zone/read-manga/claymore/
http://www.mangago.zone/read-manga/deadman_wonderland/
http://www.mangago.zone/read-manga/dogs_bullets_carnage/
http://www.mangago.zone/read-manga/dorohedoro/
http://www.mangago.zone/read-manga/fire_punch/ Angst, here I come
http://www.mangago.zone/read-manga/to_you_the_immortal/ Angst angst angst angst
http://www.mangago.zone/read-manga/fullmetal_alchemist/ (yeah okay it's a light one but daaaaaang Nina)
http://www.mangago.zone/read-manga/kinoko_takenoko/ why do they have to look so cute when the thing's dark af???
http://www.mangago.zone/read-manga/parasite/
http://www.mangago.zone/read-manga/land/
http://www.mangago.zone/read-manga/kiba_no_tabishounin_the_arms_peddler/
manga:
http://www.mangago.zone/read-manga/noragami/
http://www.mangago.zone/read-manga/rain/
http://www.mangago.zone/read-manga/ao_no_exorcist/
http://www.mangago.zone/read-manga/d_gray_man/
http://www.mangago.zone/read-manga/nanatsu_no_taizai/
webtoon:
http://www.mangago.zone/read-manga/crepuscule/
http://www.mangago.zone/read-manga/godly_bells/
http://www.mangago.zone/read-manga/tower_of_god/
Any dark themed mangas like tokyo ghoul, black butler, ajin, pandora hearts and SnK?? Need to take a rest from all the yaoi lol ( ̄∇ ̄")