The answer is simple and you have to follow some steps closely. Don’t panic, cause be sure that you are not the only one!

Step 1:
Connect your phone to your desktop in the so called ‘Mass storage’ mode or even better if you have a card reader then insert your memory card there and connect it to your desktop/laptop.

Step2:
Then your desktop recognizes your card right? So a new drive appears under ‘My Computer’? Coool! Now go ahead and open it, and venture yourself into the folder inside your memory card called — private10207114import.

Step3:
There you will see several folders and these are the folders made for your theme one by one as you installed them. 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 in their respective folder… meaning, if you want to delete a theme called “Troublesome” then simply look for “Troublesome.mbm”.

Step4:
Once you’ve found the folder containing the theme files you’d like to remove which can be tricky cause sometimes they do not name it with the theme names. In that case you have to simply open the .SKN file with Notepad or wordpad, (right mouse, open with, Scroll through and select Wordpad) and you will find the name you are looking for at the first paragraph of that file opened in Wordpad or Notepad. Done? Now just select all items of the folder, (Ctrl+A) and simply delete the contents of the folder that you selected (don’t delete the folder itself as it might not be possible).

Now if you check back the memory card inside you phone, then you will see the space difference of the theme you deleted.
Sometimes, in newer handsets you will see that theme folders/files are located in two different directories: [!:private10207114import] and [!:resourceskins]. Don’t panic, in that case simply delete the theme named (the theme you want to delete) folders from each directory to remove themes from your memory card!

Hope this helps!