Hello im new to the Odoo world and using community 10 version.  I would like to know how and if there is possible to change css to change colors etc for existing modules. For example i would like to change this modals header color.  https://i.imgur.com/JS8kfLm.png
Looking the source code i see its named  "div.modal-header" but i cant find the correct location where the css file for that modal is located. If im right it should be somewhere in Odoo\server\odoo\addons \ .... \ static \ src \ 
What is the correct way of doing it ?