Some thoughts.
AFAIK, Xcode documentation are doc sets (google
xcode doc set).
Ref:
https://developer.apple.com/library.../FindingandViewingDeveloperDocumentation.html
Doc sets have a package/bundle structure, and contain the various resources (files, sub-dirs, etc.) for presenting the docs.
The actual doc content of most Xcode doc sets is HTML. If you look inside the bundle for an installed Xcode doc set, you should be able to see the HTML resources. Try it, see what you find.
HTML can be styled using CSS. So look for the following: Does the HTML in an existing doc set contain any CSS style sheets? If so, is it embedded in the HTML files directly, or are there separate CSS style sheet files referenced by multiple HTML files?
If the HTML contains CSS, either embedded or by referencing a separate CSS file, then edit it and see if the styling change appears in the Xcode doc viewer.
If the doc set has separate CSS files, then simply edit them for all the styling changes you want. If the CSS is embedded in the HTML files, then factor it out to a file, change the HTML to reference it, and see if it works.
If the HTML contains no CSS, do an experiment to discover if the Xcode doc viewer will apply CSS styling. It can be as simple as changing the page background color of one HTML file using embedded CSS. You can make a content change at the same time, e.g. add "fnord" to the first paragraph, and confirm that the content changes.
If this works, i.e. it changes the color in doc viewer, the next experiment is to reference an external CSS style sheet from an HTML file. I'd start with a relative URL, but it might not matter.
If the external CSS style sheet works, then you can put your desired CSS style sheet(s) in a common location, either in /Users/Shared/ or in your ~/Library/ folder. I wouldn't expect the doc viewer to expand ~/ correctly, so that part could be a challenge unless you only care about one user.
If all that works, you can make a CSS file and add references to it in every HTML file of the doc set. Writing a program to add the CSS reference will probably be a better solution than editing every file manually.
The Xcode doc viewer may have a cache. If this happens, a change to an HTML file, in either styling or content, may not be immediately reflected in what the doc viewer displays. If this happens, you may have to find its cache on disk and purge it. Or maybe quitting suffices (a non-persistent cache).
The doc set may be signed. I don't know if Xcode itself would check signatures after doc sets are installed, or whether it's just the installer that would need a proper signature. If a signature is needed in an ongoing way (Xcode checks), then that's going to be a PITA. If signatures are unchecked after installation, then changes to HTML, CSS, and doc sets are simpler.
I have not tried any of this. It's just a line of thought and exploration starting from Xcode doc sets. Experimentation will be needed. Batteries not included.
