its very easy.....i have a symbian belle phone and therez a best app on softpedia which can be used to create design theme for your phone click the following link
http://www.softpedia.com/get/Mobile-Phone-Tools/Others/Carbide-ui-Symbian-Theme-Edition.shtml
Here are some key features of "Carbide.ui Symbian Theme Edition":
· Creation of graphical theme packages for all Symbian devices.
· Use of built-in and customised theme components.
· Editing of themes.
· Importing and exporting of themes.
· Viewing of all colours or only common colours in the properties view for multiple selections of Scalable Vector Graphics (SVG) images, with the Colourize feature.
· Automatic selection of a component in the Component view to match an item selected in the Editor view.
· Support for Symbian themes including 3-, 9-, and 11-piece components, with features to switch between multi-piece and single-piece components.
· Copying of theme content from the file system to apply customisation to all components of a theme simultaneously, using Edit > Paste element(s) into theme. Element IDs are resolved based on file name.
· Text colours that can be referenced and multiple text colours that can be changed in one step.
· Referenced Colours view that supports drag-and-drop features for colourising theme and SVG components.
· SVG-thumbnail previews in the File dialog box.
· Ability to switch the phone display simulation in the Editor among all supported resolutions and orientations.
· A plug-in feature that enables device-specific layouts and new customizable components to be plugged into Carbide.ui.
· Support for scoping third-party icons to a specific theme or as available to all themes. Tool bar icon to access Third party icon definitions
· Command line packaging option for themes created with any plug-ins.
following are the requirements
· 1 GB of RAM.
· 1 GB of free disk space.
· 1.5-GHz processor.
· Display supporting 16-bit colour at 1,280 x 1,024-pixel resolution.