
Make It Disappear
I'm going to skip this one, as I can't think of any circumstance under which you'd need it.

Borders
To get rid of the borders
<style type="text/css">
.new_topic, .new_topic td { border: none; }
</style>
To put a border of your own around it:
<style type="text/css">
.new_topic { border: 1px solid red; }
.new_topic td { border: none; }
</style>
[ More Border Styles ]

Background
To set a background color:
<style type="text/css">
.new_topic { background: red; }
</style>
Or a background image:
<style type="text/css">
.new_topic { background: url('http://www.PATH TO YOUR IMAGE.gif'); }
</style>
More on
[ Backgrounds ]

Styling Text
The new topic table contains both plain text and links. You can style the text, links, and even links in
their hover state all separately:
<style type="text/css">
.new_topic {
color: red;
font-family: "Times New Roman", Times, serif;
}
.new_topic a:link, .new_topic a:visited {
color: orange;
font-size: 8pt;
text-decoration: none;
}
.new_topic a:hover {
color: green;
background: yellow;
font-size: 8pt;
text-decoration: underline overline;
}
</style>
More
[ Text Styles ]

Size
You can change the width of the new topic table:
<style type="text/css">
.new_topic { width: 400px; }
</style>

Positioning
You can use margins to set a bit more space to the top or bottom, or left or right of the new topic table:
<style type="text/css">
.new_topic {
width: 400px;
margin-left: 200px;
margin-bottom: 60px;
}
</style>

Textarea, Subject & Button
To change the look and style of the textarea, subject line & button:
<style type="text/css">
.new_topic input, .new_topic textarea {
background: yellow;
color: green;
font-size: 8pt;
border: 1px solid deeppink;
}
</style>