Hi brasspla !
you can change css for login message or system message in system.css file in templates/system/css folder, find following code section at about line 18 to 34 :
/* System Messages */
#system-message { margin-bottom: 10px; padding: 0;}
#system-message dt { font-weight: bold; }
#system-message dd { margin: 0; font-weight: bold; text-indent: 30px; }
#system-message dd ul { color: #0055BB; margin-bottom: 10px; list-style: none; padding: 10px; border-top: 3px solid #84A7DB; border-bottom: 3px solid #84A7DB;}
/* System Standard Messages */
#system-message dt.message { display: none; }
#system-message dd.message { }
/* System Error Messages */
#system-message dt.error { display: none; }
#system-message dd.error ul { color: #c00; background-color: #E6C0C0; border-top: 3px solid #DE7A7B; border-bottom: 3px solid #DE7A7B;}
/* System Notice Messages */
#system-message dt.notice { display: none; }
#system-message dd.notice ul { color: #c00; background: #EFE7B8; border-top: 3px solid #F0DC7E; border-bottom: 3px solid #F0DC7E;}