#plenty #userstyle for forum:

plentymarkets-forum.css

tr.category-keyaccounting-c-9548 { background-color: #0a394a; }

div.reply-area { background-color: #43000b; }
body.category-keyaccounting-c-9548 div.reply-area { background-color: #0a394a; }

header > div.wrap { background-color: #43000b; }
body.category-keyaccounting-c-9548 header > div.wrap { background-color: #0a394a; }

.btn.create { display: none; }
.btn-flat.reply.create { display: none; }
body.category-keyaccounting-c-9548 .btn.create { display: flex; }
body.category-keyaccounting-c-9548 .btn-flat.reply.create { display: flex; }