help needed edit file from plugin

i am writing a plugin and i want it to have the possibility to change its css from a textarea in the plugin section.

I am able to code the metabox, the textarea(which loads the css file) but i cannot save it, when i press the button. The code is as follows. Please help me

add_action('add_meta_boxes', 'edit_meta_box');

function edit_meta_box() {
 add_meta_box("edit", "Editor", 'view_edit_box', "resp_edit", "normal");

}

function view_edit_box() {

	$file = stripslashes('style.css');

	$plugin_files = get_plugin_files($file);
	$file = validate_file_to_edit($file, $plugin_files);
	$real_file = WP_PLUGIN_DIR . '/' . $file;

	if( isset($_POST['newcontent']) ) {
		$newcontent = stripslashes($_POST['newcontent']);
		if ( is_writeable($real_file) ) {
				$f = fopen($real_file, 'w+');
				file_put_contents($f, $newcontent);
				fclose($f);
		}
		//echo $newcontent;
	}

	$content = file_get_contents($real_file );
	$content = esc_textarea($content);

	$html1 = '<form action="" method="post">
				<table class="form-table"><tbody><tr valign="top"><td>
				<textarea cols="85"
						  rows="80"
						  name="newcontent"
						  id="newcontent1" tabindex="1">'.$content.'
			 	</textarea>
				</td></tr></tbody>
			</table>
			<input type="submit" name="sub" value="Update File">
			</form>';
		echo $html1;	

}