I am not familiar with python's test unit, but I tried my best.

As far as I see there are three possibilities to invoke the function:
* without pattern -> return tuple with all themes
* with pattern that matches one or more themes
* with pattern that matches no themes -> return empty tuple

So I tried to add a method to test_style.StyleTest() :

    def test_themes(self):
        installed_themes = self.style.themes()
        some_themes = self.style.themes('*e*')
        no_themes = self.style.themes('foobarbaz')

        self.assertTrue(isinstance(installed_themes, tuple))
        self.assertTrue(isinstance(some_themes, tuple))
        self.assertTrue(isinstance(no_themes, tuple))

Oddly enough this fails on my own system (debian squeeze, tk-8.5.8, python-3.1.3 / -2.6.6):

$ python3 test_style.py
test_configure (__main__.StyleTest) ... ok
test_layout (__main__.StyleTest) ... ok
test_lookup (__main__.StyleTest) ... ok
test_map (__main__.StyleTest) ... ok
test_theme_use (__main__.StyleTest) ... ok
test_themes (__main__.StyleTest) ... ERROR

======================================================================
ERROR: test_themes (__main__.StyleTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_style.py", line 97, in test_themes
    no_themes = self.style.themes('foobarbaz')
  File "/usr/lib/python3.1/tkinter/ttk.py", line 536, in themes
    return self.tk.splitlist(self.tk.call('ttk::themes', pattern))
TypeError: Can't convert '_tkinter.Tcl_Obj' object to str implicitly

----------------------------------------------------------------------
Ran 6 tests in 0.086s

FAILED (errors=1)
Traceback (most recent call last):
  File "test_style.py", line 108, in <module>
    run_unittest(*tests_gui)
  File "/usr/lib/python3.1/test/support.py", line 955, in run_unittest
    _run_suite(suite)
  File "/usr/lib/python3.1/test/support.py", line 938, in _run_suite
    raise TestFailed(err)
test.support.TestFailed: Traceback (most recent call last):
  File "test_style.py", line 97, in test_themes
    no_themes = self.style.themes('foobarbaz')
  File "/usr/lib/python3.1/tkinter/ttk.py", line 536, in themes
    return self.tk.splitlist(self.tk.call('ttk::themes', pattern))
TypeError: Can't convert '_tkinter.Tcl_Obj' object to str implicitly

The same error occurs with python-2.6.6 (same tk). Apparently this is because:

$ python3
Python 3.1.3 (r313:86834, Nov 28 2010, 11:28:10) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from tkinter import *
>>> from tkinter import ttk
>>> r=Tk()
>>> s=ttk.Style()
>>> x = s.themes('foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.1/tkinter/ttk.py", line 536, in themes
    return self.tk.splitlist(self.tk.call('ttk::themes', pattern))
TypeError: Can't convert '_tkinter.Tcl_Obj' object to str implicitly
>>> x=r.tk.call('ttk::themes', 'foo')
>>> x
<StateSpec object at 0x9b1b6c8>
>>> str(x)
''
>>> 

Called from wish the same call returns an empty string as expected:

$ wish
% ttk::themes
classic default clam alt
% ttk::themes *e*
default
% ttk::themes foobarbaz
%

In python2.6, when setting Tkinter.wantobjects to 0, themes('foo') returns an empty tuple as expected.
So I guess this is due to a bug in debian's python/tkinter install. Can anyone confirm this?