If you need to uninstall Refined for Confluence, and you don't intend to use it anymore, please follow these steps.
Change to the default Confluence theme
Go to: General Configuration > LOOK AND FEEL > Themes, and change the global theme to the default theme.
This step will prevent Confluence to look for a theme that is not there anymore
Uninstall the add-on
Go to: General Configuration > Manage add-ons > Expand Refined Theme and click uninstall add-on/app.