/* This changes color of the entire sidebar column section */

div#sidebar {
background-color:#222222;
}

/* This changes color of the entire signup button surrounding area */

.sidebar-box {
background-color:#222222;
}

/* This changes color of the "Sign Up Now!" */

.buttons a, .buttons button { 
background-color:#222222;
}

/* This changes color of the outline around button */

buttons a, .button a, button {
border:2px solid #222222 !important;
}

/* This changes color of the "Not A Member" */

div.sidebar-box h3 {
color:#FFFFFF;
}

/* This changes color of the background inside button */
#sb-signup .buttons a, #sb-signup .buttons {
color:#FFFFFF;
}

/* This changes color of the "Sign In" surrounding area */

form fieldset, .fieldset {
background-color:#222222;
border-color:#222222;
}

/* This changes color of the "Sign In" Heading */

div#sb-signin h3 {
color:#FFFFFF;
}

/* This changes color of the "Sign In" field labels for Email and Password */

div#sb-signin form label {
color:#FFFFFF;
}

/* This changes color of the "Keep me signed in" */

div#sb-signin td {
color:#ffffff;
}

/* This changes color of the "Forgot Your Password?" */

div#sb-signin a:link, a:visited, a:hover, a:active {
color:#FFFFFF;
}

SocialGO Signup and Login CSS Elements and Attributes

CSS