This function is defined in the ttk initialization script ttk.tcl.
This function takes a theme name as an argument. It looks to see if the theme has been loaded, and if not, requires the package, and calls ttk to use the theme. While the code actually also sets a variable, that variable is local to the function and thus isn't available.
Note that X11 users can set the X resoruce database to specify a preferred theme (use *TkTheme: themeName), and if one is not provided, ttk uses the "default" theme.