Do you have existing visualforce pages created, and want to have Lightning experience look and feel or CSS?
Don't worry, as part of Summer 18 Salesforce has come up with an attribute for it.
To style your Visualforce page, set lightningStylesheets="true" in the <apex:page> tag.
All editions will have this feature available.
Note: This feature will enable lightning CSS in lightning experience and not in Salesforce Classic.
