WordPress Admin Thickbox: Remove Margins/Padding

You can use this for a custom stylesheet. Copy and paste all the CSS from the original thickbox.css in your custom stylesheet.

Now you can edit your custom CSS and add the code below in your plugin.

function get_thickbox() {
    // Add the original thickbox
    add_thickbox();

    // register your custom stylesheet
    // Make sure its in your speciefied folder!
    wp_register_style( 'thickbox_css', plugins_url( 'pluginname/stylesheet.css' ) );

    // enqueue our new stylesheet
    enqueue_style( 'thickbox_css' );
}
add_action('wp_enqueue_scripts', 'get_thickbox');

// Thats it!
// You can now reach your customized thickbox by:
get_thickbox();