Board Thread:Support Requests - Getting Technical/@comment-44745594-20200913055258/@comment-45117243-20200913063801

On your wiki, I think you never loaded the DarkTheme.js through any of the default-loaded JS pages (common.js, Wikia.js, importJS).

About the error - I'm not familiar enough with JS, but it seems that you have a ! operator applied on the function, while you are not assigning this value to anything.