How can I change the background of a certain topic?

Development
8

You would need to style the published page separately from the topic itself, but every published page has its own ID so it is possible to tweak them with some CSS. eg:

.published-page {
  &.testing-published-page.topic-276.foo {  /*this is my page ID which is composed of topic-title+topic-ID+category-slug*/
    background-color: #c4ddc4;

    .published-page-header {
      background-color: #facccc;
    }

    blockquote {
      background-color: #facccc;
      border: 5px solid #fb7878;
    }
  }