You can now control the style of the composition background using the full range of QGIS' symbology options. It's now possible to export compositions with a transparent (or semi-transparent) background. Shape items (rectangles, triangles and ellipses) can also be styled using the same options as polygon map layers. You can even style the page background or shapes by using data defined settings based on the current atlas feature! There's also a new option for rounding the corners of rectangle shapes.