Yes, the "look" of the timer is definitely customizable any way you'd like. To modify the timer graphics (colors, textures, etc.), start with the default graphics file, which can be downloaded from the following URL:
Download default graphics file
The file looks like this:
(The image shown here is JPG for browser display only, you should download the TGA version for editing.)
Once you've downloaded the default file, you can open it up in a graphics program and make modifications to it. As long as the graphical elements stay in the same locations, the timer XML will load up your graphics correctly.
Modifying the timer graphics is definitely encouraged! If you create a timer skin that you'd like to share (see Tipop's skins for excellent examples), send it to me and I'll post it on the user-submitted skins
section of the timer site.