How do I make the topic list more compact?

Product Support
12

This should cover what you highlighted. Feel free to adjust based on the original value.

.topic-post {
  .topic-body {
   padding-top: 0;  /* 0.8em */
  }
  
  .cooked {
     padding-bottom: 0;  /* 1em */
  }
  
  .post-menu-area {
    margin-top: 0; /* 20px */
    margin-bottom: 0; /* 20px */
  }
}