You can try OwnSkin: Free Nokia Themes, Nokia Theme Creator, SonyEricsson Themes, Mobile Themes
*Tip: Though is a longer method but you can get it absolute free
You download the themes to your computer then you transfer to your phone instead of smsing the code to download it.
And since you're using SE phone, you can actually dont need to type in SE c903, you can just randomly choose any SE phone models and you will be able to get what you want