On PowerPC Macs like the PowerBook you should be able to use
ThemeChanger + a DLTA theme for free. Keep in mind that this is actually swapping GUI resource files out (like CandyBar does with icons) rather than applying a theme to programs as they open (ShapeShifter).
Tikiug (
direct download) should let you convert most themes into a format you can use in ThemeChanger. GuiKitty (ShapeShifter 1.x themes only) +
Guikit2DLTA used to be the standard way under OS 10.3 of extracting a ShapeShifter guikit's resources, but they've been superseded by Tikiug.