Texture overlay on Header image

Hi Alex! I added an header background image by adding one in styles.css at the #header class. Now i want to lay a texture png over this image. But when I add an additional div to the header area in the header.php, it covers not the whole header area. How to add an additional div covering the complete header? (As an uppermost item but excluding the main menu)

Thank You so much!

Best Regards,

Sascha