There are couples of ways to download and install theme on your mobile phone.
Open the web browser on your mobile phone and go to www.drikpanchang.com/themes.html and click download button after choosing your device and follow the instructions on your phone and you're done.
If you have downloaded the theme on your PC or laptop then there are two ways to install it on your mobile phone. One is through the Nokia PC suite and the other is through data cable transfer. Here are the instructions
Connect your mobile phone via USB cable and select "data transfer" on the mode menu. When you connect your mobile phone, it shows up as a removable disk on your PC or laptop. You can transfer the theme from PC or laptop to your phone's storage card. You can remove the data cable and install the theme by using the file manager (in build application on mobile phone under office folder) and selecting the theme to install.
If your friend has theme on his or her mobile phone, you can ask your friend to transfer/send it on your mobile phone via Bluetooth. The theme will appear on your messages and to install it, simply click on the transferred file on your messages and follow the instructions on your phone and you're done.
Please check following settings on your mobile phone if you get installation error. Please make sure that 'software installation' is set to either 'on' or 'all' and 'online certificate check' is set to 'off'.
Software installation = ON or ALL
Online certificate check = OFF
Depending on your mobile phone you can find above settings under
Menu > Tools > App. Mgr. > Options > Settings
Menu > Ctrl. Panel > App. Mgr.> Options > Settings
Menu > Application Manager > Options > Settings
Menu > Settings