Add a background to tw-passage
tw-passage {
background-color: rgba(255, 255, 255, 0.75);
border: 2px solid white;
padding: 5px;
min-height: 80vh;
}
Border and passing are optional, but help with making sure the text doesn't end up right at the edge of the box. The min-height setting makes the box look a bit larger even on passages with little content.