In the olden days, moodle used to include some documentation in the main download, as well as the more context-specific help files. (These days, all the documentation lives on docs.moodle.org.)
docstyles.php were some extra styles that would be used only on the documentation pages. But since those pages no longer exist, docstyles.php is obsolete.
There are still some references to docstyles.php in some of the themes. They need to go.