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Suppose you want to add a footer with the following two lines of text at the end of each page that is shown on the right side:
© 2017 - My Company Ltd
All rights reserved

For this, you need to specify a rule that matches all the right pages and adds that content in the bottom-center. In your customization CSS, add the following CSS rule:

@page :right{
    @bottom-center {
        content: "© 2017 - My Company Ltd \A All rights reserved";
        font-size: 0.5em;
        color: silver;
    }    
}
Note: Other page rules (such as the table-of-contents) override the contents of the @bottom-center because they are more specific. If you need to also print the copyright in the TOC pages, then use this as the selector:
@page :right, table-of-contents:right {
  ...
}
Note: To use new lines (\n characters) in your headers or footers, use the \A notation, as in the example above.