`Theme <https://pytablewriter.readthedocs.io/en/latest/pages/reference/theme.html#pytablewriter.style.Theme>`
consists of a set of style filters.
The following command will install external predefined themes:

::

    pip install pytablewriter[theme]

Themes can be set via the constructor of the writer classes or the ``set_theme`` method.
The following is an example of setting the ``altrow`` theme via the constructor.
``altrow`` theme will be colored rows alternatively:

:Sample Code:
    .. code-block:: python

        import pytablewriter as ptw

        writer = ptw.TableWriterFactory.create_from_format_name(
            "markdown",
            headers=["INT", "STR"],
            value_matrix=[[1, "hoge"], [2, "foo"], [3, "bar"]],
            margin=1,
            theme="altrow",
        )
        writer.write_table()

:Output:
    .. figure:: https://cdn.jsdelivr.net/gh/thombashi/pytablewriter-altrow-theme@master/ss/ptw-altrow-theme_example_default.png
       :scale: 100%
       :alt: https://github.com/thombashi/pytablewriter-altrow-theme/blob/master/ss/ptw-altrow-theme_example_default.png

`[theme]` extras includes the following themes:

- `pytablewriter-altrow-theme <https://github.com/thombashi/pytablewriter-altrow-theme>`__
    - `Generated HTML table example <https://thombashi.github.io/pytablewriter-altrow-theme/example.html>`__
- `pytablewriter-altcol-theme <https://github.com/thombashi/pytablewriter-altcol-theme>`__
    - `Generated HTML table example <https://thombashi.github.io/pytablewriter-altcol-theme/example.html>`__
