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To modify the styling of the default page headers, add the following CSS rule in your customization CSS:

@page :left {
    @top-left {
        color:navy;
        font-style:italic;
    }
    @top-right {
        color:red;
    }
}

If you intend to modify just the headers of the table of contents, use the table-of-contents page rule selector:

@page table-of-contents:left {
    @top-left {
        color:navy;
        font-style:italic;
    }
    @top-right {
        color:red;
    }
}