theming not documented in customizing

[mattfarina]

When theming recently landed (https://review.openstack.org/#/c/159300/) the customizing section of the docs was not updated to reflect this. See https://github.com/openstack/horizon/blob/4fdc42cf6332579c162de57ec5ec833343bff3e5/doc/source/topics/customizing.rst

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/166989

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/166989
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=a9c6b52f69c83369e8c4fd6b46ab615aecfdf756
Submitter: Jenkins
Branch: master

commit a9c6b52f69c83369e8c4fd6b46ab615aecfdf756
Author: Matt Farina <email address hidden>
Date: Mon Mar 23 15:54:11 2015 -0400

    Update customizing docs to include themes

    Themes for horizon were recently introducted in
    https://review.openstack.org/#/c/159300/. This
    change updates the documentation on customizing
    to include themes.

    Change-Id: I48d04008a44e4ac7efe46d3ec3fbbdc15a55258b
    Closes-bug: #1435384