In Message Composer, in order to get the right look you must use various Design Blocks. For example you may have a footer that has a Divider block a color background, and then a text block with an image background, another text block with a different background, and then a link bar block. Together they make one visual entity, and rather than saving each block and pulling them in separately into an email, it would be great to save the disparate blocks into one content super block.