If you encounter graphene - Stylesheet is missing issue after updating the theme to version 1.9.2, delete graphene directory from wp-content/themes using file manager/FTP. Then install the Graphene theme again at Appearance --> Themes --> Install Themes.
Why is this happening?
Quoting Syahir's reply,
This happens sometimes when WordPress is sending incomplete package to your server or the zip extraction during update didn't go smoothly.
Will I lose any customizations done to the Graphene Theme if I delete it from themes directory?
Any modifications done using Graphene Options will be preserved. Deleting Graphene theme will not erase Graphene Options stored in the database.
You can download Graphene 1.9.2 from here and manually upload the theme to WordPress. Instructions are available in that page.