Do you want to change the font for all the fields, or just for the sender. and have different fonts for read and unread? Here is an example that changes the colour, size and weight for unread:

/* unread card sender */
tr[is="thread-card"][data-properties~="unread"] :is(.sender) {
      color: black !important;
      font-weight: bold !important;
      font-size: 14px !important;
   }

The font family can also be set by including something like:

font-family: Times, serif !important;

Hi. I suppose for the sender only would suffice. What is irking me that the font remains in bold or as in bold (due to a type of font chosen by the developers) for read and unread emails please see screenshot. The top email is unread. The one below has been read, yet the font has not changed. I find that confusing. Many thanks.

Here is the change I am after. Screenshot from the Thunderbird in Windows.

You can override bold by changing it to normal in the css.

Each element of the card, subject, sender, date, can have separate colours, size, weight etc.

/* indent card subject */
.thread-card-subject-container :is(.subject){
	margin-left: 10px !important;
	font-size: 13px !important;
	color: darkred !important;
}

/* card date */
.date{
	font-size: 12px !important;
	color: darkred !important;
}

Thanks boss! Worked like a charm.