Go to 'Admin' > 'Layout' and check off this option:
Custom HTML at top of every page:
In the text area below it, add this code and modify the text in bold:
<style>
.qa-widget-main h2:first-of-type {
background: #9b59b6;
}
</style>
Alternatively, adjust this line of code in qa-theme/SnowFlat/qa-styles.css:2833