file.scala

scala> import net.liftweb.util.Helpers._
import net.liftweb.util.Helpers._

scala> val html = <div class="domnav">
     |   <div class="header">
     |     <p>Paragraph for header</p>
     |   </div>
     |   <div class="main">
     |     <h2>Title main</h2>
     |     <p>Paragraph for main</p>
     |     <div class="left">
     |       <h3>Title left</h3>
     |       <p>1st paragraph for left box</p>
     |       <p>2nd paragraph for left box</p>
     |     </div>
     |     <div class="right">
     |       <h3>Title right</h3>
     |       <p>1st paragraph for right box</p>
     |       <p>2nd paragraph for right box</p>
     |     </div>
     |   </div>
     |   <div class="footer">
     |     <p>Paragraph for footer</p>
     |   </div>
     | </div>

scala> val css = ".domnav .header p *" #> "Diego!!!"
css: net.liftweb.util.CssSel = CssBind(Full(.domnav .header p *), Full(ClassSelector(domnav,Empty)))

scala> css(html)
res0: scala.xml.NodeSeq = 
NodeSeq(<div class="domnav">
  <div class="header">
    <p>Diego!!!</p>
  </div>
  <div class="main">
    <h2>Title main</h2>
    <p>Paragraph for main</p>
    <div class="left">
      <h3>Title left</h3>
      <p>1st paragraph for left box</p>
      <p>2nd paragraph for left box</p>
    </div>
    <div class="right">
      <h3>Title right</h3>
      <p>1st paragraph for right box</p>
      <p>2nd paragraph for right box</p>
    </div>
  </div>
  <div class="footer">
    <p>Paragraph for footer</p>
  </div>
</div>)

scala>