I'm sorry for beeing too hasty, I solved it myself. I added this line above template:

<xsl:variable name="siteRoot" select="$currentPage/ancestor-or-self::*[@level = 1]" />

And added this before the for each loop:

      <!-- Home node added here -->
      <li>            
        <a href="{umbraco.library:NiceUrl($siteRoot/@id)}">
          <xsl:if test="$currentPage/@id = $siteRoot/@id">
            <xsl:attribute name="class">active</xsl:attribute>
          </xsl:if>
          <xsl:value-of select="$currentPage/ancestor-or-self::*/@nodeName"/>            
        </a>        
      </li>

This is the final code of my top navigation menu which highlights the home link:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet [ <!ENTITY nbsp "&#x00A0;"> ]>
<xsl:stylesheet
  version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:msxml="urn:schemas-microsoft-com:xslt"
  xmlns:umbraco.library="urn:umbraco.library" xmlns:Exslt.ExsltCommon="urn:Exslt.ExsltCommon" xmlns:Exslt.ExsltDatesAndTimes="urn:Exslt.ExsltDatesAndTimes" xmlns:Exslt.ExsltMath="urn:Exslt.ExsltMath" xmlns:Exslt.ExsltRegularExpressions="urn:Exslt.ExsltRegularExpressions" xmlns:Exslt.ExsltStrings="urn:Exslt.ExsltStrings" xmlns:Exslt.ExsltSets="urn:Exslt.ExsltSets"
  exclude-result-prefixes="msxml
 umbraco.library Exslt.ExsltCommon Exslt.ExsltDatesAndTimes 
Exslt.ExsltMath Exslt.ExsltRegularExpressions Exslt.ExsltStrings 
Exslt.ExsltSets ">

    
    <xsl:output method="xml" omit-xml-declaration="yes" />
    <xsl:param name="currentPage"/>
    <xsl:variable name="level" select="1"/>
    
    <xsl:variable name="siteRoot" select="$currentPage/ancestor-or-self::*[@level = 1]" />

    <xsl:template match="/">
      <!-- Home node added here -->
      <li>            
        <a href="{umbraco.library:NiceUrl($siteRoot/@id)}">
          <xsl:if test="$currentPage/@id = $siteRoot/@id">
            <xsl:attribute name="class">active</xsl:attribute>
          </xsl:if>
          <xsl:value-of select="$currentPage/ancestor-or-self::*/@nodeName"/>            
        </a>        
      </li>
    
    <!-- Sub nodes added here -->
    <xsl:for-each select="$currentPage/ancestor-or-self::* [@isDoc and @level=$level]/* [@isDoc and string(showInNavigation) = '1']">
      <li>
        <a href="{umbraco.library:NiceUrl(@id)}">
          <xsl:if test="$currentPage/ancestor-or-self::*/@id = current()/@id">
            <xsl:attribute name="class">active</xsl:attribute>
          </xsl:if>
          <xsl:value-of select="@nodeName"/>
        </a>
      </li>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

I hope someone will find this useful.

Maybe an admin want to check this thread as solved? :)

is working on a reply...