design Joomla 1.5 template

For a client we are building a website (including a simple webshop feature), in Joomla! 1.5. Since I have been used to create the graphic design for Dreamweaver application, thinking Joomla is a challenge considered the time-span available for this project. Building the website is not so much the problem, it is the template-design that is not yet part of my skills. The whole site will be ok with two templates, the only difference is the colour-scheme and a few graphics. There are no how-to-build-a-template-for-Joomla! 1.5. - manuals out there to help me out, only for 1.0 and that doesn't work. Take a look at the layout to get an idea of what I wish to accomplish, I can provide you with graphics, hex colours, dimensions etcetera. There is sort of a time-challange here, so estimated time needed to create one (or two) templates would be appreciated.