Use the z-index property:

.background {
  position: absolute;
  z-index: -9999;
}

I’ve followed this answer to get this html character as a background image. It works well but not as a background, the HTML character is on top of the text. I already tried several posibilities but I think there’s something out of my knowledge.

.number {position: relative;
    font-family: sans-serif;
    font-size: 18px;
    font-weight: bold;
    color: red;
}
.number::after {
    content: "2617"; 
    color: blue; 
    margin-left: -1em; 
    font-size:30px;
}

<div class="number">
90
</div>