doc: Use "console" for the highlight language of console output

Using "console" language is better for the console example (including
command-line prompt)

doc/usage/theming.rst

@@ -70,7 +70,7 @@ directory containing :file:`conf.py` and use this configuration::
 The third form is a Python package.  If a theme you want to use is distributed
 as a Python package, you can use it after installing
 
-.. code-block:: bash
+.. code-block:: console
 
     # installing theme package
     $ pip install sphinxjp.themes.dotted