Looks up a named icon and returns a gtk.IconInfo containing information such as the filename of the icon. The icon can then be rendered into a pixbuf using IconInfo.loadIcon. (IconTheme.loadIcon combines these two steps if all you need is the pixbuf.)
When rendering on displays with high pixel densities you should not use a size multiplied by the scaling factor returned by functions like Window.getScaleFactor. Instead, you should use IconTheme.lookupIconForScale, as the assets loaded for a given scaling factor may be different.