1. To edit the website using html go to the "Website" then click on the "Customise" link in the top right corner.

2. You can change the existing .css by accessing web folder in the addons. You can also place inline stylings as well as inline js. Changing the original css is not recommended as after update it will disappear and it is against the rules (inherit and override rather than change).
3. As mentioned above please read:
\https://www.odoo.com/documentation/9.0/
The topic on overriding CSS:
https://www.odoo.com/forum/help-1/question/override-css-and-change-styles-of-some-fields-44128