To use css in odoo create a folder static->src->css-> then create file_name.css.

and in the xml,

<?xml version="1.0" encoding="utf-8"?>
<openerp>   
 <data>      
     <template id="assets_backend" name="project assets" inherit_id="web.assets_backend">
            <xpath expr="." position="inside">     
                 <link rel="stylesheet" href="/field_highlight/static/src/css/field_name.css"/>
            </xpath>       
    </template>  
 </data> 
</openerp>