It is easy to customize the look and feel of the ZK XUL components by providing your own style sheets (CSS). You can refer to norm.css.dsp and normie.css.dsp in the zul project for examples. If you just want to change the font size, you can reference normsm.css.dsp.

<desktop-config>
    <theme-uri>/css/my-fine-font.css</theme-uri>    
</desktop-config>