here is a CSS i have made for it however you can change the image in it to whatever you want as it's not very good

body {
background-image: linear-gradient(to bottom, rgba(0,0,0,0) 100px, rgba(115,124,190, 0.9)), url(https://endchan.org/.media/5b0ae810d804fc903ec57813475d16c5-imagepng.png);
background-size: 620px;
background-repeat: repeat;
background-attachment: fixed;
}

.input-group-addon, .label, .btn, .nick-hover, #plmeta, #videowrap-header, #chatheader, #newmessages-indicator-bghack {
background: #f2f3f7;
color: orange;
border-color: #ff6600;
}

#mainpage {
color: orange;