There is a section of TEI under the "Guidelines" menu item called Customization.

Issue 1813 https://github.com/TEIC/TEI/issues/1813  points out some problems that are eminently fixable. However, what Syd and I were unable to determine is where the source lives (other than on disk) and how to edit the file(s). 

Help appreciated. 
Signing off as the September Issue Assigner.
best, --elli