gecko-dev/browser/base/content/aboutDialog.css

 #aboutDialog {
   padding: 0px 0px 10px 0px;
   width: 299px;
 }

 #clientBox {
   background-image: url("chrome://branding/content/about.png");
   background-repeat: no-repeat;
   background-color: #FFFFFF;
   padding-top: 203px;
   color: #000000;
 }

 #brandName {
   font-weight: bold; font-size: larger;
 } 

 #detailsBox {
   padding: 8px 10px 0px 8px;
 }

 #userAgent {
   margin: 3px 10px 0px 5px;
   background-color: #FFFFFF;
   color: #000000;
   padding: 1px 0px 0px 3px;
   -moz-appearance: none;
   overflow: hidden;
   border: 0px;
 }

 #groove {
   margin-top: 0px;
 }

 #creditsIframe {
   cursor: default;
   -moz-user-select: none;
   border: 0px;
 }

 #version {
   font-weight: bold;
   color: #909090;
   margin: 1em 0px 10px 17px;
 }

 #copyright {
   margin: 0px 0px 3px 16px;
 }
 
 button[dlgtype="extra2"] {
   margin-left: 13px;
 }
 
 button[dlgtype="accept"] {
   margin-right: 13px;
 }