You are using the Website Wizard for creating your site. So the answer to your questions are:
1. You can't.
2. You can't.
The Website Wizard is intended for beginners who don't know, or don't want to know, anything about HTML and web site design. There is nothing wrong with this, but in order for the automatic generation of web pages to work, much of the web page components are not accessible to the developer. The CSS file and the
tag inaccessibility prevent the inclusion of a background image, and the automatic indexing of pages prevents the editing of the navigation menu.
If you want to do more than the Website Wizards allow, upload a template and use the editors in the File Manager. Start with a simple one and build from there. Do not try to mix the File Manager and the Website Wizards. They are not compatible and you risk overwriting you web page designs. You can not upload a template into the Website Wizard.