Mybb Editor toolbar please help!

[Firetech]

I found out the problem it was actually really easy to fix lol but I have another problem if you would like to help with that I leave the info in this spoiler

Spoiler (Click to View)

Okay so I'm trying to edit the style of my editpost.php but I don't know where the file is..

Okay so as you can see on this image I want to change the size of the sqaure you type in and I can find the place with google chrome and edit as I like it but when I find the right settings I can't find it in the admin panel or in my folder I simply don't know what the file I'm looking for is named anyone know how this works please help me thanks