Create a flexible box

Unless you are going to support older browsers like Internet Explorer 8 (IE8), you should use CSS3 to create rounded rectangles. The tutorial on this page "Create rounded corners text/image element using CSS" will show you how to do this.
We are going to create a rounded corners flexible box which allows us to change it's size without creating new image for each. This method only works on background with sold colors. This cannot be applied to background with gradient colors.