put an enclosing <div></div> around your textboxes, and use CSS to set the backcolor.
You should always have a style sheet to set common display colors etc..
You put a link tag in the head section of your aspx page like so.
<head>
...
<link rel="stylesheet" type="text/css" href="Styles.css">
</head>
You enclose your textboxes in a div with a classid (colored in this case) like so:
<div class="colored">
<asp:textbox id="123" runat="server" />
<asp:textbox id="124" runat="server" />
</div>
You add a style-sheet to your project (add new item... style sheet.. call it 'Styles.css'),
then add a style identifier for your class ('colored').
in the style sheet:
div.colored
{
padding: 20px;
width: 400px;
border: solid 1px #000000;
background-color: #555555;
}
Save all, and run.