Sorry I am a bit new to MyBB still and well, I can't find a way to fully edit the html to make certain html coding take in different elements so I can add in my own css bit to them. I just want to know where i can change the div classes from an html bit like the thread view or the forumlist... etc...
To Add CSS:
ACP > Style & Templates > Themes > Your Theme > Global.css
Edit that file and you can place all the css there.
To Add HTML:
ACP > Style & Templates > Template > Your Template > Index
Index is the main file to edit. if you wanna edit the header then open the header file. Pretty much everything is in ACP > Style & Templates > Template > Your Template, for Advance HTML & PHP & JavaScript & any other language.