You have to create a new module with a view to inherit from `web.login_layout` template from `web/views/webclient_templates.xml`. So

• Create a new module `mytheme`:
• `__init__.py`: Empty, because we do not need to import any module
• `__openerp__.py`:

{
    'name': 'My Theme', 
    'version': '0.1', 
    'depends': ['web'], 
    'external_dependencies': {},
    'data': ['webclient_templates.xml'],
    'js': ['static/src/js/mytheme.js'],
    'css': ['static/src/css/mytheme.css'], 
    'installable': True,
    'auto_install':False,
    'active':True,
}

•  `views/webclient_templates.xml`:

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>
        <template id="assets_backend" name="mytheme assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">
                <link rel="stylesheet" href="/mytheme/static/src/css/mytheme.css"/>
                <script type="text/javascript" src="/mytheme/static/src/js/mytheme.js"></script>
            </xpath>
        </template>
        <template id="mytheme.login_layout" inherit_id="web.login_layout" name="My Theme Login">
            <xpath expr="//div[@class='oe_single_form_footer']" position="replace">
                <div class="oe_single_form_footer">Here you can write your login footer</div>
            </xpath>
            <xpath expr="//div[@class='oe_single_form_logo']" position="replace">
                <div class="oe_single_form_logo">
                    <img src="/mytheme/static/src/img/logo.png"
                         alt="My Theme Logo"
                         title="My Theme Logo" />
                </div>
            </xpath>
        </template>
    </data>
</openerp>

• Create CSS and JS files:
  • static/src/css/mytheme.css
  • static/src/js/mytheme.js
• Copy your logo (180x90 px is a good one):
  • static/src/img/logo.png

Install module in Settings > Modules > Local Modules. This will override login template, you can inherit from any other template and add or replace any html using an xpath expresion.