Add style files that can be imported into documents

For example:

MyStyles.xml:
<?xml version="1.0" encoding="utf-8"?>
<Styles>
    <Style Target="Paragraph">
        <Setters Format.SpaceBefore="2mm" Format.SpaceAfter="2mm"/>
    </Style>

    <Style Target="Paragraph" Name="Title">
        <Setters Format.Font.Bold="true" Format.Font.Underline="Single"/>
    </Style>
</Styles>

Invoice.xml:
<?xml version="1.0" encoding="utf-8"?>
<Document>
    <IncludeStyles Path="MyStyles.xml"/>
</Document>