This applies to Windows 7 with Aero. I don't know how much applies to other systems.
It seems there are three parts to "theming" spark.
1. The Theme:
Choose between several windowing themes which change some of the inner gui elements as well as the windowing elements (minimize, title bar, etc.). If you choose "Windows" it will use system look and feel for these elements.
There is no way to use system look and feel for the window, without also changing the internal elements. In contrast, other chat programs may let you skin the internals while using a system look and feel for the window--this is neither good nor bad, just a difference.
2. The color preferences. This allows you to change colors of certain elements such as text, lists, both foreground and background.
This is practically useless from a customization point of view because:
a. It takes a while to fiddle with the colors.
b. The colors only change certain things. For example, you can change the text color of buddies in your contact list, but you can't change the group colors (blue). So if you wanted a black background, it would only apply to the buddies, and leave the groups in a contrasting blue color--which looks rather poor.
c. You cannot reset the colors to defaults. This makes you want to not touch these colors for fear of getting something illegible and not remembering what you changed.
d. You have to 'apply' each color changed, before changing to the next element, or it will discard your changes.
e. It doesn't select the color as you tab to each field, forcing you to first delete the old color, then type a new one.
f. The colors don't change immediately, but only after you hit save. But then, they still don't always change in the contact list, forcing you to restart spark.
3. The "Spark" theme, which seems unchangeable. Inner backgrounds of the windows are either blue or gradient blue, with no clear indication why. In particular, the chat window has a flat blue background for the contact name/avatar and chat window theme sections. This color is not very pleasing, particularly if you are using any windows theme which doesn't contrast well with it. Other backgrounds seem forced to white, although this may be changeable in the colors from (2).
I would like to be able to change (3) in particular. It would be nice if a theme could control all 3, though. But at least an option to turn off that blue background which seems to have replaced what was once an unnoffending white (not my favorite, but it didn't stick out).