Here are some code snippets to help you change the look of your size chart.

You can put them in your theme.liquid file.

/* change the font and border style of the whole table */
.scr-modal table td {
  font-family: Arial;
  border: 1px solid black;
/* make the first row black with white, centered text */
.scr-modal table tr:first-child td {
  background: black;
  color: white;
  text-align: center;
/* give the rows after the first row a white background and center the text */
.scr-modal table tr:nth-child(n+2) td {
  background: white;
  text-align: center;