Hiya
Just to add to what's been said in this thread already, the API rendering section is not done by native Drupal code. It's done by a reactjs application called apiconnect-explorer. It is also used in API Manager, Consumer Catalog, AppConnect, CP4i, Tivoli, etc..
I am afraid you can't customise the output of that reactjs application, you can obviously apply custom SCSS on top though.
If you have UX improvements you'd like us to make to the code out of the box (that would benefit everyone) then please do let me know!
Cheers
Chris
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Chris Dudley
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