CSS itself cannot disable an HTML tag. You need to set the HTML tag to disabled ro readonly. To style an input tag, please see below.
<html>
<head>
<title>Disable Textbox</title>
<style>
.read[readonly], .disa[disabled] {color: #fff; background-color: #666;}
</style>
</head>
<body>
<form>
<input type="text" class="disa" disabled />
<input type="text" class="disa" />
<input type="text" disabled />
<input type="text" class="read" readonly>
</form>
</body>
</html>