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.