Sorry, we cannot achieve that only with css.
You will need to use a program such as Adobe Fireworks to create a white background image with low opacity, in the form of a png file. Then apply it to the navbar as the background. Don’t set any color value to it.
Please contact us via our services page https://theme4press.com/theme-customization-service/ or hire a developer from http://jobs.wordpress.net/ to assist you in this customization.