Set the type to "Other/Manual" and then use following code:
<?php
$options = ['fa-angle-up' => '<i class="fa fa-angle-up"></i>', 'fa-chevron-up' => '<i class="fa fa-chevron-up"></i>', 'fa-window-close' => '<i class="fa fa-window-close"></i>'];
return new \IPS\Helpers\Form\Radio("core_theme_setting_title_{$row['sc_id']}", $value, FALSE, array(
'options' => $options,
'parse' => "raw"
), NULL, NULL, NULL, 'theme_setting_' . $row['sc_key']);
Result:
You could also use the IPS\Helpers\Form\Custom form field type which allows you to literally customize the whole output...