Portamx have a full support for the SMF core theme, but I can't say if it works with other themes like "core".
If you have your theme in a other folder, the PortaMx can't detect if is a "core" theme. You can modify the detection in PortaMx.php around the line 65 where you find:
// check if old core theme
$context['pmx_style_isCore'] = (bool) (stripos(str_replace('\\', '/', $settings['theme_dir'] .'/'), '/core/') !== false);
If you have ONLY core themes, you can set $context['pmx_style_isCore'] = true; else you have to make more checks like my one...