Full style sheet for the "Yellow Box" Twine:

body {
    background-color: yellow;
     color: White;
    background-image: url("http://wallmanage.com/wp-content/uploads/2016/01/best-sailboat-hd-desktop.jpg");
    background-repeat: no-repeat;
    background-position: right top;
    margin-right: 200px;
    background-attachment: fixed;
}
p {
    color: White;
}
tw-passage {
    width: 60%; padding: 2em; margin: auto; border: solid #000 0.05em; border-radius: 0.2em; font-family: Geneva, "Helvetica Neue", Helvetica, sans-serif; font-size:1.5rem; color:#000; text-align:left; background-color:#fff; box-shadow: #000 0.5em 0.5em 0;
}
 
tw-link {
color: rgb(255, 0, 215); padding: 7.5px; border: 2px solid gray;
}
 
mark {
font-family: Papyrus; transform: rotate(90deg);
}
 




This can set the background image, if you paste it in the stylesheet editor within twine:
 body { background-image: url("https://43e98e4dea9cc16d999b-4ff86151a1684680afb5e7ba0d0f4374.ssl.cf2.rackcdn.com/VWKi2CjeAy_1444594238706.jpg?rasterSignature=4db13bce21106d5782ab9aa19a91f297&theme=Five%20Seven%20Five&imageFilter=false"); }
body




This can be pasted directly on a page:
<span style="font-size: 22px; color: #F0FFF0; ">