Hi,

yes is not the good way ,do not modify the original code.

it is the same principle,work with inheritance.

create a new module . in this module create your file.css in folder static/src/css .

your_module/static/src/css/custom_skin.css

and in __openerp__.py add this line :

'css' : ["static/src/css/custom_skin.css", ],

Example if you want change color of menu to red :

@charset "utf-8";
.openerp .oe_menu > li > a {
    color: red;    
}

I recommend you use Firebug to know the class css.