How to add a custom css?

What is the best way to add custom css to openerp 7 without modifying any original file of openerp?

Yes it will not work in odoo 8, in odoo 8 you need to inherit the css template in your module and include your css file there e.g

<template id="my_assets_id" name="website assets for my module" inherit_id="website.theme">
        <xpath expr="." position="inside">
        <link rel="stylesheet" href="/website_my_module/static/src/css/my.css"/>


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