This question has already been addressed numerous times!
Think of the structure of your document. A different page background means a different logical part. This is reflected by using a different page style.
For basic information about styles, read the relevant chapters in the Writer Guide.
Once you have the page style(s) for your various book parts, you have to assign the required page style to these parts.
CAUTION! You don’t assign a page style to a page (because pages don’t exist per se; they are only the consequence of text flow). You tell Writer how to lay out text when it needs a new page. The new page is defined by the page style (margins, columns, background, …).
When you are at the boundary between your parts, you insert a “special” page break. The usual Ctrl+Enter or Insert>Page Break is not sufficient because it only request a page break without changing page properties.
Instead, use Insert>More Breaks>Manual Break. In the dialog, you can choose which page style will format the pages after the break. You can also reset the page number if desired.
A page style is effective between two “boundaries”. A boundary is start of document, special page break or end of document. All pages between boundaries are formatted the same.
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