A theme with the name "CustomMySite 1011" and version already exists on the server.

You receive this error when trying to apply a custom theme to your SharePoint site:

"A theme with the name "CustomMySite 1011" and version already exists on the server. "

1. Open your site in SharePoint Designer.
2. Expand _themes and delete the CustomMySite theme folder
3. Re-apply the custom theme to your site.