rakashmiri , it is relatively easy to change themes for the 3.5.xx series. Just remember to always backup your original firmware, and to always use your own firmware for theming purposes.
By using other's firmwares you will only kill your player.
First you'll need to extract your current firmware, which is also your backup. The guide is
Here.
Next, is modifying your firmware images. The guide is
Here.
There are many themes in the downloads section, but not all will work with your firmware version as the sizes and layout might be different. However, you can manually edit the images to suit your firmware.
When done, upload your firmware back into your player. The guide is
Here.