When using the ​Custom Fields feature, there are two conflicting CSS styles.

More specifically the background-color in the following CSS bit:

#postcustomstuff table {
  margin: 0;
  width: 100%;
  border: 1px solid #dcdcde;
  border-spacing: 0;
  background-color: #f6f7f7;
}

with the background-color in the following CSS bit:

.alternate, .striped > tbody > :nth-child(2n+1), ul.striped > :nth-child(2n+1) {
  background-color: #f6f7f7;
}

resulting in all rows of the table in div#postcustomstuff to have the same background color (#f6f7f7)