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;
            }

            #chatwrap {
                background: #f2f3f7; 
                border: 1px solid #ff6600; 
                border-left: none; 
                border-top: none;
                color: orange;
            }

            .userlist_owner {
                color: red !important;
            }

            #chatline {
                background: #b7c5d9 !important;
                color: orange;
                border-color: transparent !important;
            }

            input.form-control[type="text"] {
                background: #f2f3f7 !important;
                border-color: #ff6600 !important;
                color: orange;
            }

            .userlist_op {
                color: orange !important;
            }

            .well {
                background: #f2f3f7;
                color: orange;
                border-color: #ff6600;
                border-width: 1px;
            }

            .well strong, .poll-notify {
                color: #ff6600;
            }