Update:
If you don't need to change the style at runtime, you can be set it in the .azw2 manifest. This is also working without the jailbreak.
Quick example from an ant buildfile:
Code:
<manifest>
<attribute name="Implementation-Title" value="${product.name}"/>
<attribute name="Implementation-Version" value="${product.version}"/>
<attribute name="Implementation-Vendor" value="${product.vendor}"/>
<attribute name="Main-Class" value="${product.mainclass}"/>
<attribute name="Amazon-Cover-Image" value="${product.image}"/>
<attribute name="Toolbar-Style" value="none"/>
</manifest>
Allowable values are "none", "transient", and "persistent".
More interesting information about supported Kindlet attributes can be found in the com.amazon.kindle.kindlet.internal.install.Metadat a class