add this at the end of the swagger's css file

swagger.dark.css

body.swagger-body {
    background: #3b4151;
}

h1, h2, h3, h4, h5, h6, pre {color: #fff!important;}

.swagger-ui .scheme-container {
    background: #3b4151;
}

.swagger-ui .opblock.opblock-post {
    border-color: #49cc90;
    background: rgba(73,204,144, 0.6);
}

.swagger-ui .opblock.opblock-get {
    border-color: #61affe;
    background: rgba(97,175,254, 0.6);
}
.swagger-ui .opblock.opblock-put {
    border-color: #fca130;
    background: rgba(252,161,48, 0.6);
}
.swagger-ui .opblock.opblock-delete {
    border-color: #f93e3e;
    background: rgba(249,62,62, 0.6);
}
.swagger-ui .opblock.opblock-patch {
    border-color: #50e3c2;
    background: rgba(80,227,194, 0.6);
}
.swagger-ui .opblock-tag {
  color: #fff
}