Thanks for your anwser but in N80 it's not like that... I've found the anwser and it's like this :

1. Connect your device to your PC in 'Mass storage' mode - or insert your Mini SD card into a card reader and connect it to your PC.

2. Once your PC recognizes your card a new drive will appear under 'My Computer'. Open it, and browse to private10207114import.

3. Browse through each of the folders in this directory to find the theme you would like to remove. The MBM, MIF, and SKN files in each folder should each be named after the theme - so if you want to remove a theme called "MyTheme", look for "MyTheme.mbm" etc.

*Note that you may see some MBM/MIF/SKN files named simply "themepackage.mbm" etc. In this case, simply open the SKN file in Notepad and you will see the actual theme name in the first few lines of text.

4. Once you've found the folder containing the theme files you'd like to remove, simply delete the contents of the folder (don't delete the folder itself).

Now put your card back in your phone you'll notice that the theme is no longer listed under themes and you've freed up some space on your memory card.