To customize the post date format you're going to look for this code in your template:
h2.date-header {
The CSS inside the brackets {} following that code will control what your post date look like.
You can customize the code however you'd like--be creative! You can use my CSS Basics posts on Borders, Backgrounds, Text, and Padding & Margins for specific codes. If you have any questions on how to do something specific, ask in a comment and I'll get back as soon as I can.
For example, my current post title code looks like this:
h2.date-header {
font-family: Perpetua;
color: #70543e;
font-size: 14px;
font-style: italic;
float: left;
padding-top: 44px;
padding-left: 75px;
padding-bottom: 0px;
margin-bottom: -10px;
}
font-family: Perpetua;
color: #70543e;
font-size: 14px;
font-style: italic;
float: left;
padding-top: 44px;
padding-left: 75px;
padding-bottom: 0px;
margin-bottom: -10px;
}