See XML in Excel and the Spreadsheet Component for the full documentation. But it often helps to just create a spreadsheet, save it as *.xml and review the output. Example:
...
<Styles>
<Style ss:ID="s23">
<Interior ss:Color="#800080" ss:Pattern="Solid"/>
</Style>
<Style ss:ID="s24">
<Interior ss:Color="#0000FF" ss:Pattern="Solid"/>
</Style>
</Styles>
<Worksheet ss:Name="Sheet1">
<Table ss:ExpandedColumnCount="1" ss:ExpandedRowCount="3" x:FullColumns="1"
x:FullRows="1">
<Row>
<Cell ss:StyleID="s23"><Data ss:Type="String">apple</Data></Cell>
</Row>
<Row>
<Cell ss:StyleID="s24"><Data ss:Type="String">orange</Data></Cell>
</Row>
</Table>
</Worksheet>
...