@Anick: Sorry I don’t have any recommendation for follow/subscription box. Maybe you can find good plugins at http://wordpress.org/plugins/
Category is different and it has it’s own id. So, your “Les a-cotes” category ID is 21. So, the css will be as below:
.category-21 #site-generator { background-color: #696969; }
If you are finding difficult to find the IDs then you can use our simple plugin https://wordpress.org/plugins/catch-ids/