<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="1.0">
  
  <xsl:output method="html" indent="yes"/>
  
  <xsl:param name="bg1" select="'red'"/>
  <xsl:param name="bg2" select="'green'"/>
  <xsl:param name="bg3" select="'blue'"/>
  <xsl:param name="bg4" select="'yellow'"/>
  
  <xsl:template match="sec[not(ancestor::sec)]">
    <div style="border: 1px solid {$bg1};">
      <xsl:apply-templates/>
    </div>
  </xsl:template>
  
  <xsl:template match="sec/sec">
    <div style="border: 1px solid {$bg2};">
      <xsl:apply-templates/>
    </div>
  </xsl:template>

  <xsl:template match="sec/sec/sec">
    <div style="border: 1px solid {$bg3};">
      <xsl:apply-templates/>
    </div>
  </xsl:template>

  <xsl:template match="sec/sec/sec/sec">
    <div style="border: 1px solid {$bg4};">
      <xsl:apply-templates/>
    </div>
  </xsl:template>
  
</xsl:stylesheet>

<xsl:template match="sec">
  <xsl:variable name="depth" select="count(ancestor::sec)+1">
  <div class="div{$depth}">
      <xsl:apply-templates/>
  </div>
</xsl:template>