I'm a big fan of Nokia Series 40 phones, but the one problem I have with them is that they all look very similar.
If you fancy customizing your phone to stand out from the crowd a bit, you could install a theme for your device.
However, those who really dare to be different choose to create their own themes and the Series 40 Theme Studio allows you to do just that.
Series 40 Theme Studio - an official Nokia software package - lets you change virtually every part of your display, from the folder icons to the battery indicator. You can tweak colors, change fonts and add custom pictures to set your phone apart from the rest.
The user interface in Series 40 Theme Studio is split into two halves - the left containing a menu of all the things you can change, and the right a live preview window displaying the current state of the theme.
The degree you can customize your device really is limited only by your imagination. Actually, it's also limited by your patience because the user interface in the Series 40 Theme Studio isn't very user friendly. The menus are cluttered and poorly-labeled and this program is crying out for drag-and-drop functionality.
Note that if you have a newer Nokia phone not support by the Series 40 Theme Studio, then go check out the Series 60 Theme Studio, which will let you create themes for S60 phones.
If you have the persistence to master the cumbersome interface in Series 40 Theme Studio, you should be able to pull off some cool designs though.