Last updated: 2018-11-14
Works with CiviCRM 4.7 and should work on CiviCRM 5.0 or higher
The "Style Guide" extension demonstrates common HTMTL/CSS styling conventions used within CiviCRM. It adds a new submenu, "Support => Developer => Style Guides" with a list of demo pages.
For more information, see https://github.com/civicrm/org.civicrm.styleguide.