Userscripts.org

Have a look at this:
http://www.hunlock.com/blogs/Totally_Pwn_CSS_wi...

You could use something like that, create a custom css entry for all the elements you want and add a css style rule for "input and submit", you could control the background color, border anything you want as you would normally via css....

for reference see:
http://www.w3schools.com/Css/default.asp

define input, play around with anything you want:
border, color, font-size, font-family...

Examples:
/* === Form Elements === */

form {
margin: 0;
padding: 0;
border: 0;
}

input {
color: #FFCC33;
background-color: #111111;
border: 1px solid #CCCCCC;
font-family: Verdana, "Trebuchet MS", "Lucida Grande", Helvetica, sans-serif;
font-size: 1.1em;
font-weight: normal;
padding: 1px;
}

textarea {
color: #FFCC33;
background-color: #111111;
border: 1px solid #CCCCCC;
font-family: Verdana, "Trebuchet MS", "Lucida Grande", Helvetica, Arial, sans-serif;
font-size: 1.3em;
line-height: 1.4em;
font-weight: normal;
padding: 2px;
}

select {
color: #FFCC33;
background-color: #111111;
border: 1px solid #CCCCCC;
font-family: Verdana, "Trebuchet MS", "Lucida Grande", Helvetica, sans-serif;
font-size: 1.1em;
font-weight: normal;
padding: 1px;
}

input:hover, textarea:hover, select:hover {
color: #DD2222;
background-color: #232323;
border: solid 1px #FFCC33;
}

input:focus, textarea:focus, select:focus {
color: #FFCC33;
background-color: #111111;
border: solid 1px #DD2222;
}

option {
padding: 0 1em 0 0;
}