Today Opera software has released a Halloween edition of Opera browser for Windows and Mac.

You might want to check out the other cool entries here.

How to install if you are already using Opera:

If you already have Opera, you need to backup 3 files and rename them in order to get the Halloween experience:

1. Open menu “Help” » “About Opera”.
2. Find the path to the catalog with user data (“Path” » “Preferences”) and copy the address
3. Close Opera
4. Open the catalog with user data found above (for Windows: “Start” » “Run” » paste the address you copied before)
5. Remove or rename the following files:
   • bookmarks.adr
   • speeddial.ini
   • operaprefs.ini
6. Install the customized version (without removing the previous one)

Source: Opera Community blog