It’s possible that either your font size was already 18px, or that your theme isn’t great at cascading down font size values. Try adding this big font-size value to custom.css, and see if things get huge and ugly.
BTW, at least on MacOS, the CSS changes should take effect within a few seconds, no restart required. But if you don’t see the changes right away, a restart might help.
:root {
--ct-text-size: 32px;
--ct-line-height: 1.6;
}