To customize the logo appearing on the top left of the screen, as admin you can go in Setting > Configuration > General Setting then click on "Configure your company data". Then you can click on "Edit" and change the logo then save. The new logo will effectively appear on the top left after you reload the page (press F5).

If that is not sufficient, and you also want to completely change the appearance and color of OpenERP, the best way would be to add a css file in OpenERP (if you know css). This can be achieved by creating a new module with a css file and installing it. The module can be extremely simple, with just a __openerp__.py file and the css file.

Example __openerp__.py:

{
'name' : 'Agrolait Design',
'version': '1.0',
'summary': '',
'sequence': '18',
'category': 'Tools',
'complexity': 'easy',
'description':
    """
    """,
'depends' : ['base'],
'css': ['static/src/css/agrolait.css'],
'installable': True,
'auto_install': False,

}

Then you just need to create the static/src/css/agrolait.css file.