How do I change the placeholder text styling in Squarespace forms?

Placeholder text uses the ::placeholder pseudo-element. You need to include vendor-prefixed versions for cross-browser support, though modern browsers mostly support the unprefixed version.

Keep placeholder text lighter than input text so users can distinguish between a filled field and an empty one. A 40-50% opacity of your text color works well.

.form-item input::placeholder,
.form-item textarea::placeholder {
  color: #999999 !important;
  font-style: italic;
  font-size: 14px !important;
  opacity: 1;
}

/* Webkit browsers */
.form-item input::-webkit-input-placeholder,
.form-item textarea::-webkit-input-placeholder {
  color: #999999 !important;
  font-style: italic;
}