Use Appearance → Customize → Additional CSS. Rules added there survive theme updates, which edits to style.css do not.
Finding the right selector
Right-click the element in your browser and choose Inspect. Modern Magazine uses readable, prefixed class names, so the selector you see is usually the one to target.
Common tweaks
/* Change the body font size */
body { font-size: 15px; }
/* Hide the footer credit line */
.copyright a { display: none; }
/* Constrain a very wide logo */
.logo img { max-height: 60px; width: auto; }A note on the bundled fonts
As of 1.2, Modern Magazine bundles PT Sans locally under /fonts/ rather than loading it from a remote host. If you want a different typeface, either enqueue it from a child theme or override font-family in Additional CSS — do not edit the bundled font CSS, as an update will replace it.