You are no longer able to edit the themes like we used to do in 4.x version. If you want to add something into a template you can make it by using "Theme Designer - Templates" section.
To edit the header position "menu, userbar, search bar, logo, etc" and things such as fonts, color, background, you can make it via the "Theme Editor" accesibile from the forum.
This is the path
/system/Theme/Theme.php
check for something like this:
$calledClass = get_called_class();
if( $functionName == 'theme_core_front_global_footer' or ( $functionName == 'footer' and $app == 'core' and $location == 'front' and $group == 'global' ) )
{
$content = $content . "\n<p id='elCopyright'>
<span id='elCopyright_userLine'>{lang=\"copyright_line_value\"}</span>
{{if !\$licenseData = \IPS\IPS::licenseKey() or !isset(\$licenseData['products']['copyright']) or !\$licenseData['products']['copyright']}}<a rel='nofollow' title='Community Software by Invision Power Services, Inc.' href='http://www.invisionpower.com/'>Community Software by Invision Power Services, Inc.</a>{{endif}}
</p>";
}
and remove the
{{if !\$licenseData = \IPS\IPS::licenseKey() or !isset(\$licenseData['products']['copyright']) or !\$licenseData['products']['copyright']}}<a rel='nofollow' title='Community Software by Invision Power Services, Inc.' href='http://www.invisionpower.com/'>Community Software by Invision Power Services, Inc.</a>{{endif}}
Cheers