In order to use styling in the email, like bold, you must use HTML.
And to use HTML you must use a special setting named htmlBody.
Please study the documentation at the following link:
The HTML must be "inline". So you can't use style tags and class names.
The user interface is a different issue, that would take more time to explain.
I haven't addressed that issue here, if someone wants to, please feel free.
I've provide some code that should get you started on the bold line that you want.
function reminder() {
var list,mailSettings,mensaje,row,tarea;
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sheet = ss.getSheets()[0];
var dataRange = sheet.getRange(2, 1, sheet.getLastRow(), 1);
var data = dataRange.getValues();
var heading = "<b>This is a reminder of things that need to be done:</b><br/>";
list = "";
for (var i in data) {
row = data[i];
tarea = row[0];
list += tarea + "<br/>";
}
mensaje = heading + list;
var miCorreo = "
mye...@gmail.com";
mailSettings = {};
mailSettings.to = miCorreo;
mailSettings.subject = "Tasks reminder";
mailSettings.htmlBody = mensaje;
MailApp.sendEmail(mailSettings);
}