Isn't it this CSS in static/css/base.css? Also, If you have any interest in CSS, Chrome/Safari have an "Inspect Element" command and Firefox has the terrific Firebug add on. These are all must-have tools for working with web page styles.
/*********** web2py specific ***********/ div.flash { font-weight: bold; display: none; position: fixed; padding: 10px; top: 40px; right: 10px; min-width: 280px; opacity: 0.85; margin: 0px 0px 10px 10px; color: #fff; vertical-align: middle; cursor: pointer; background: #000; border: 2px solid #fff; -moz-border-radius: 5px; -webkit-border-radius: 5px; z-index: 2; } div.error { background-color: red; color: white; padding: 3px; }