Or maybe just replace the images in the copied theme?
Instead of copying the theme in the control panel, another option is to download it, then replace the images before uploading it again. This would prevent the theme from being upgraded 'automatically', though.
I copied the theme, go to Site settings -> Theming -> Modify Theme -> rclick on barceloneta-favicon.ico
this offers "Upload Here" -> browse -> select a local file with my favicon
The upload window reports 'File upload failed'
Is there a way to determine why the upload failed?
Upload fails with a security warning
Secure Connection Failed PR_CONNECT_RESET_ERROR
On your file-system, in the folder I mention, you will find the favicon.
If you replace it here, it will show for the theme.
NOTE: Some browsers cache the favicon, so check with another browser if it does not change.
The reason I mention 'src' is because your favicon will be replaced if you upgrade the theme later. If you are SURE you will remember to replace the favicon the next time you upgrade, it is probably OK to do the changes 'like that'.
If you go to your Plone folder on the file system and search for 'favicon' (or barceloneta-favicon), you should find it.
The favicon does NOT exist (as an image) in the ZODB, it is ONLY on the file system.