Having detailed conventions for public members of libraries is important, because they affect many developers, are hard to change and people often use multiple libraries at the same time, so consistency is important.
On the other hand, the conventions for non-public members affect much smaller number of developer (only those working on your code), are easier to change (much less worry about breaking changes) and people usually work on a single codebase at a time. This means having extensive official guidelines is not really necessary.
If you want to follow some style guide, you can find a project which publishes its guide. For example, here is the one for corefx.