I wanted to make the indivitual post pages 100% wide, and also the headings a bit bigger. Here's the CSS to do it:

/* Make individual posts full-width */
.singular.page .hentry {padding:1em;}
.singular .entry-header,
.singular .entry-content,
.singular footer.entry-meta,
.singular #comments-title {
 width: 100%;
}
/* Fix the edit button placement */
.singular .entry-meta .edit-link a {
  bottom: auto;
  left: 0px;
  right: auto;
  top: 30px;
}
/* Make headings a bit bigger */
.singular .entry-content h1 {
  font-size: 28px;
  margin-top: 44px;
}
.singular .entry-content h2 {
  font-size: 18px;
  margin-top: 28px;
}