fembooru/themes/danbooru2/ext_manager.theme.php

<?php

class CustomExtManagerTheme extends ExtManagerTheme
{
    public function display_table(Page $page, array $extensions, bool $editable)
    {
        $page->disable_left();
        parent::display_table($page, $extensions, $editable);
    }

    public function display_doc(Page $page, ExtensionInfo $info)
    {
        $page->disable_left();
        parent::display_doc($page, $info);
    }
}