Start menu? Do you mean the date in scripted text above? That I changed with TinkerTool (Again free). Unfortunately it doesn't apply to everything on the Finder in Leopard(not sure about Tiger), but it does transfer over to other applications. You can choose whatever font you like. It also tweaks some other stuff, which can be useful.
If you meant the "wood" background for my Finder windows, with a window open on the Desktop you go to View --> Show View Options and then at the bottom of that window will be "Background" and you can select a color or a picture. I used a generic wood picture I found using Google. Make sure that is BIG as it has to fill the entire window part of it. At least 2000pix, or you'll have space that is the usual white color. It won't tile it for you, sadly. I'd do this in for your main hardrive and then click "Use as Defaults" so that it will apply for all of your Finder windows.
If you want to change your icons the easiest way is outlined here:
http://pimpmymac.wordpress.com/2007/03/05/how-to-change-icons/. The nice thing about this way is that you can just take plain 'ol jpeg or png files, copy the picture and use it as the application or folder picture. Like that gear I used for my main hard-drive (two posts previous). That's a jpeg, not a custom made icon.
Main icons like the Finder and Trash can only be changed using LiteIcon or Candybar (which isn't free), and that application needs .icns for it to work. But it's really simple and I like it. Candybar also simplifies the changing of applications to just dropping .icns files into the image well. But again, it isn't free, and it won't always work, especially with multi-icon applications (won't work at all with iCal).
However, some have multilayers or "packages" that make up one application such as iCal or Adium. Then you have to right click the application and select "Show Package Contents". Then you have to find all of the icons for that application and replace the main ones. They are usually in the "Resources" folder. You don't have to change them all, just the ones that show up on your dock and in your Applications folder. You can find out more about how to change them online. This harder way REQUIRES .icns files and you can't just cut and paste pictures. If you have Developer, use the Icon Composer as it's just great!
Again, I would always back up the original icons as you never know when that application could take a hiccup. I've probably typed WAY more than you wanted to know, but hopefully it was helpful.