That linked approach describes how you had to do it for two rounded corners for a fixed height. An image for the left side with rounded top, a 1px wide image for the background with the content, and a right side image with rounded corners at the top.

What you had to do for a rounded popup was a 3x3 grid. 4 images for the top left/right, bottom left/right. A repeating 1px image for the top middle, 1px for the bottom middle, 1px for the vertically repeating left, 1px vertically repeating right, then finally center element that was generally a solid color.

No flexibility on color because of, at the very least, the corner images.

Things are so much simpler now.