I am currently theming the new intranet for my company. To say that this is a learning process is to put it lightly. I started from the top of the page and have been styling down. As part of the process, I have been trying to understand the whole layout of the masterpage, how all the pieces are put together, and all the styles that make up each part. I have two monitors side by side, with Sharepoint Designer open on one and Internet Explorer on the other. I use the IE Developers toolbar to help me find the styles that are being used on a particular element. I have the theme stylesheet open as well as core-sometimes you have to grab from core, so keep it handy. If I am having trouble “seeing” the layout of an area of the page, I copy the code from view source or Designer into Dreamweaver and i explore it there-I find it easier to see the table structure in DW than Designer. I also preview my css in DW because I prefer how DW works compared to Designer, but thats just me. I also have Fireworks open for creating new versions of the images used in the theme. I am not storing the new images within the content database-we have a separate share on our network where all the SP-related images are stored. This process has been working well for me. I have been writing down everything I figure out, with my intention of typing it all up and releasing it on the site soon. Anybody else have a process they want to share?