It's not super easy to create your own custom template. However, when you edit the CSS files and change around images I find it easy to make a site look original. Play with the module position also.
there is no particular software to design a template.we need to customize the joomla file.if we need to change the color or background.We need to change template.css in css/template folder. there is a tool but it does not support fully to design a template .
see video tutorial, but it is in Russian language - try search "Ð¢ÐµÑ…Ð½Ð¾Ð»Ð¾Ð³Ð¸Ñ Ñ€Ð°Ð·Ñ€Ð°Ð±Ð¾Ñ‚ÐºÐ¸ шаблонов Ð´Ð»Ñ CMS Joomla 1.5 (Семенов Д., 2009)"
Artisteer should be good for a starter, but with a lot of creativity you can design super templates - just by using Photoshop and Artisteer!