I have added new Sub Tabs Under The Settings->Main Tab in The Silverstripe 3.0 Back-End using the following code:

public function updateCMSFields(FieldList $fields) {
       $fields->addFieldToTab('Root.Main', new TabSet('Tab1'),'Theme');
       $fields->addFieldToTab('Root.Main.Tab1', new Tab('SubTab1',new HTMLEditorField("Text", "Text")));
       $fields->addFieldToTab('Root.Main.Tab1', new Tab('SubTab2', new DropdownField("DropDown1","DropDown",
       array(
         
            '-1' => 'Option1',
            '1' => 'Option2',
            ))));
    }

But The Main Tab already has a field named theme under it which is generated by default. I need to keep this field separated from the tabs which I have added...maybe creating a new sub-tab for it...Please tell me how I can achieve this...Currently it appears under every tab...