I just thought I'd point out this is only useful if you have styles defined in a specific topic. If you just want to use your existing stylesheet in another project you can copy it using Windows Explorer. If you do need the export functionality, hopefully someone else can chip in with where it's located in RH2015, as I'm still on RH11.
I believe Amber is spot on. Looks like this function does not exist anywhere in the RoboHelp UI that I can see. It also doesn't exist as an additional command that can be added to the Quick Access Toolbar.
Finding the CSS in the project folder is simple though. Just right-click somewhere in the Project Manager and choose Explore.
Looks like that was overlooked in the help system.
Thank you - another author asked and I could not find an answer.