Java Synth XML: Button Style properties won't work

I'm new to XML - and new to Java Synth LAF...

I have loaded the following XML style file correctly to my UIManager:

<synth>
<style id="default">
    <font name="Segoe UI" size="16"/>
</style>
<bind style="default" type="region" key=".*"/>

<style id="button">
    <opaque value="true">
        <insets top="10" bottom="10" left="10" right="10"/>
        <state>
            <color type="BACKGROUND" value="#FFFFFF"/>
            <color type="TEXT_FOREGROUND" value="#000000"/>
        </state>
    </opaque>
</style>
<bind style="button" type="region" key="Button"/>

The default style works fine but the button style doesn't work for some reason.

Thanks in advance

The opaque element - Synth File Format

The opaque element is empty elements because it is declared with the category keyword EMPTY.

<!ELEMENT opaque EMPTY>
<!ATTLIST opaque
          value              (true|false)   "true"
>

<synth>
  <style id="default">
    <font name="Segoe UI" size="16"/>
  </style>
  <bind style="default" type="region" key=".*"/>

  <style id="button">
    <opaque value="true"/>
    <insets top="10" bottom="10" left="10" right="10"/>
    <state>
      <color type="BACKGROUND" value="#FFFFFF"/>
      <color type="TEXT_FOREGROUND" value="#000000"/>
    </state>
  </style>
  <bind style="button" type="region" key="Button"/>
</synth>