Here are some other places (you don't have to use the built-in ones):
When you take the "Architecting & Deploying Splunk" class you get a file called
SplunkIconLibrary.ppt; I have this file (as do many others) but I am unsure of Splunk's policy/care of posting/distributing it.
Then this should be there. Try it on any other Splunk server. I have splunk installed on my laptop, so the actual full url is
http://localhost:8000/en-US/static/docs/style/style-guide.html. I confirmed that this is valid on 6.5.2.