Just create a module and add your css file in __openerp__.py as described in the documentation.

 # __openerp__.py
 {
     'name': "CSS Example",
     'description': "new css example",
     'category': 'Generic Modules/Others',
     'depends': ['web'],
     'data': [],
     'css': ['static/src/css/my_css.css'],
 }

If you are using Odoo v8, you must add this

        <template id="assets_backend" name="mymodule css assets" inherit_id="web.assets_backend">

            <xpath expr="." position="inside">

                <link rel="stylesheet" href="/my_module/static/src/css/my_module_css_file.css"/>
            </xpath>
        </template>

if you need to write custom css in frontend odoo then you please try (1) , in case of backend try (2):

1:
    <template id="id" name="your name" inherit_id="website.assets_frontend">
      <xpath expr="." position="inside">
           <link href="/youraddon/static/src/css/yourcssfilename.css" rel="stylesheet" type="text/css"/>
      </xpath>
    </template>
2:
     <template id="id" name="your name" inherit_id="website.assets_backend">
         <xpath expr="." position="inside">
             <link href="/youraddon/static/src/css/yourcssfilename.css" rel="stylesheet" type="text/css"/>
         </xpath>
     </template>

Try as like this. It worked fine for me:

<template id="report.style" name="style">

<style type="text/css">

body {

color: #000 !important;

}

.background {

background-color: blue;

}

.p {

font-size: 22px;

color: green;

}

<!-- More of CUSTOM CSS here -->

</style>

</template>

Watch on YouTube : https://www.youtube.com/ How to add css file in odoo 10

Or simply you can use style attribute 

<p style="color:green;margin:0;font-weight:bold/* write your css instead of my example*/">
     Write something
<p>

Here is a Module that can help easily implement Custom CSS and Custom JS to Groups and Actions. 
​
https://www.odoo.com/apps/modules/11.0/custom_cssjs

​

thank you Laurent Destailleur for answer.

It work for me !

That doesn't work for me too on Odoo.

Neither with the css directly in my module nor inheriting template

https://github.com/odoo/odoo/issues/4359

The right answer is here: http://www.odoo.yenthevg.com/adding-static-resources-css-and-javascript-to-odoo/

I think that this does not work in odoo 8.0, isn't it?!

Looking for solution> trying to make vartical labels in tree view.

In my case, css is loaded but I cant make it work ... :(

Whatever I change in css I can't see it in tree view.