I found this on a website, and made a few minor improvements to it. For those of you who prefer firefox over Safari, putting this in userContent.css (/Library/Application Support/Firefox/Profiles/[Profile]/chrome) will give you round submit buttons
button, input[type="reset"], input[type="button"], input[type="submit"] {
background-color: #f1f4f7 !important;
background-image: url(aguaButton.png) !important;
background-repeat: repeat-x !important;
background-position: 50% 50% !important;
color:#101011 !important;
-moz-border-radius: .45em;
padding: .07em .25em !important;
border: 1px solid #c1c6cb !important;
border-bottom: 2px solid #8c9298 !important;
font-size:10px !important;
width:auto !important;
height:auto !important;
}