After you upgrade from Joomla! 1.5 to 1.7, the old template of Joomla! 1.5 can not be used anymore and you have to reinstall the template is compatible with the new version of Joomla. But if you want to use your old template with Joomla! 1.7, you can try converting them to be used with Joomla! 1.7

Convert a simple template for Joomla! 1.5 to 1.7

Extract your template package and browse to the template's folder.

1. Open the file named index.php

Find:

defined('_JEXEC') or die('Restricted access');

Replace with:

defined('_JEXEC') or die;
$app= JFactory::getApplication();

Find:

$mainframe->getCfg('live_site')

Replace with:

$app->getCfg('live_site')

Find:

$mainframe->getCfg('sitename')

Replace with:

$app->getCfg('sitename')

Save when done.

2. Open the file named templateDetails.xml

Find:

<!DOCTYPE install PUBLIC "-//Joomla! 1.5//DTD template 1.0//EN" "http://www.joomla.org/xml/dtd/1.5/template-install.dtd">
<install version="1.5" type="template">

Replace with:

<!DOCTYPE install PUBLIC "-//Joomla! 1.6//DTD template 1.0//EN" "http://www.joomla.org/xml/dtd/1.7/template-install.dtd">
<extension version="1.7" type="template" client="site">

Find:

<params>

Replace with:

<config>
   <fields name="params">
      <fieldset name="advanced">

Find:

<param name="...." ....>

Replace with:

<field name="...." ....>

Find:

</params>

Replace with:

</fieldset>
   </fields>
</config>

Find:

</install>

Replace with:

</extension>

Save when done.

3. Open the file named template.css in css folder

Find:

.item-separator

Replace with:

.item-separator

Find:

.blog_more

Replace with:

.items-more

Save when done.

4. Add more css if needed

.actions {
}
.actions li {
}
table.category {
}
table.category th,table.category td {
}
table.category thead {
}
.jcat-children ul,.jcat-children li {
}
.jcat-children li a {
}