Here is a really cool retro dialog box popup designed by rémi and coded by Thibaud Goiffon. It’s got a very simple look with a colourful scrolling background and is in pure CSS.
If you are having trouble with the pen, try the archived copy on GitHub
Retro Dialog Box Snippet
See the Pen rémi’s pop-up by Thibaud Goiffon (@Gthibaud) on CodePen.0