Board Thread:Support Requests - Getting Technical/@comment-33867758-20200527031900/@comment-9605025-20200527034910

It is changeable but that requires fixing a bug with the Lua module. I have gone ahead and done that so now you should be able to customize the appearance as desired. Simply add the template parameters "bordercolor" and "bgcolor" to the template's use of Template:MessageBox to change the color of the left border and the background respectively.