OS X is a very attractive operating system; however, it has received some criticism for having a relative lack of customization options. While Windows and Linux have had fairly detailed options for adjusting overall themes and individual colors and fonts used, OS X has kept its visual elements fairly standardized.
The only options Apple offers for changing themes are in the Appearance section, where you can choose between the default Blue or Graphite options for buttons, menus, and windows. In addition, you can change a few other features such as placement of scrolling buttons, text highlight color, scrolling behaviors, and font smoothing, but these are relatively minor details.
Beyond these settings, you can of course change the desktop image, dock items, and placement of menu extras to the right in the system menu, but these are more or less standard configuration options. Apple keeps more extensive configuration settings hidden to keep the OS more standardized, but you can access and modify them using a utility like TinkerTool.
TinkerTool will read the various configuration files for system components such as the Finder, the Dock, Safari, iTunes, and general system settings, and present you with all the known hidden options that are available for them. While you cannot create saved themes with TinkerTool, you can go in and customize OS X to the greatest extent possible.
Keep in mind that while you can adjust some system settings with TinkerTool, some settings (especially fonts) may result in odd-looking displays. The OS X UI Elements are built around the Lucida Grande font (between 10pt and 13pt size) and if you change this to another font you may see system text run into itself, off the edge of windows, or be hidden or otherwise obstructed by other UI elements.
Regardless of the potential drawbacks, it is at the very least fun to see how the system looks with alternate settings. If you decide to revert your changes, TinkerTool has two options for this, which are available in the TinkerTool application menu or in the Reset section in the application. The first is to revert the settings back to the way they were before you used TinkerTool, which keeps any changes you made using the system preferences. The second option is to restore all settings to their default values.
Do you have a favorite system-tweaking utility? If so, then let us know about them in the comments.