dont rely on platform specific encoding

bumblebee/theme.py

@@ -109,7 +109,7 @@ class Theme(object):
 
         if os.path.isfile(themefile):
             try:
-                with open(themefile) as data:
+                with open(themefile, encoding="utf-8") as data:
                     return json.load(data)
             except ValueError as exception:
                 raise bumblebee.error.ThemeLoadError("JSON error: {}".format(exception))