No CSS in ACP

[Spitfire_]

what is your forum url ?
can you run file verification tool available at tools & maintenance section of admin panel to find missing / changed files.
ignore files reported from install folder & its subfolders. you may also ignore reported images.
you have to replace reported files from MyBB source files pack of your forum's version.

(if you are using Google SEO plugin then do not replace ~/inc/functions.php file)

Thanks for the quick reply, I actually fixed the problem after replacing some files missing which I found from the file verification tool.