You might have probably turn off the Advance Mode which is why you are not seeing the border and box shadow options in the button settings. Please go to X > Settings > Permissions > User Preferences > Advance Mode and set it to "Always One".
Go back to your page in Cornerstone, click the button and in the button settings, you should be able to set the borders and box shadow of the button.
Hope this helps. Kindly let us know.