After selecting a layout, you can further customize it.

We can modify the current layout and content by:

Editing buttons are the top in the blue bar.

The Format button has a list of visual styles that can be applied. You can change the look and feel of things you insert with the other menu.  For example, you can change colors of backgrounds to dark grey or black.

Mosaic Editor Menu Bar: Format Menu Expanded
 

The Insert button is used for adding new Tiles to the layout.  Once you select a Tile, you can drag and drop it into the page layout.

 Mosaic Editor Menu Bar: Insert Menu Expanded