about you text box

The one in profile under information tabs?

I'm not sure whether this will affect other areas but you can paste the following in EXTRA.CSS template obviously styling it to your preference with your preferred colour.

#info .secondaryContent {
    background: url("styles/default/xenforo/gradients/category-23px-light.png") repeat-x scroll center top #F0F7FC;
    border-bottom: 1px solid #D7EDFC;
    padding: 10px;
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