Hi @al777 welcome to the forum! The styled container is not an image, but an html div with styling specified by css. You can inspect its css properties by right-clicking on it inside your browser and choosing “Inspect element”, and you will be able to explore its css (at the bottom left of the attached screenshot).
Also the source code of this app is hosted on https://github.com/plotly/dash-sample-apps/tree/master/apps/dash-oil-and-gas