Just add custom CSS to your document. You can place this line at the top (after the yaml part):
1. Inline Style
<body style="background-color:grey;">
2. Embedded Style Sheet
<style>
body { background-color: grey; }
pre, pre:not([class]) { background-color: red; }
</style>
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…