Ooofff! Last time I tried to do this, the CSS Tidy program that wordpress uses on user CSS stripped all the stuff out. The red for the month is actually a CSS gradient, and that requires several background declarations (and a filter for Internet Explorer) and CSS Tidy and/or wordpress was not allowing multiple background declarations.
Add this to the bottom of your CSS and edit the hex color codes. The first color code is the top color and the second is the bottom color. If you save this and it all basically gets stripped out, then go onto the second bit of CSS below which will allow you to set a non-gradient background color of your choice. The first line, the background with only a single color declared, is the fallback for browsers that do not recognize gradients.
.post-date .month {
background:#cb151b;
filter:progid:DXImageTransform.Microsoft.gradient(startColorstr='#eb1c24', endColorstr='#a30c10');
background:-webkit-gradient(linear, left top, left bottom, from(#eb1c24), to(#a30c10));
background:-moz-linear-gradient(top, #eb1c24, #a30c10);
background:linear-gradient(-90deg, #eb1c24, #a30c10);
}
If the above gets stripped by CSS Tidy/wordpress, add the following to the bottom of your CSS and set the single color you want.
.post-date .month {
background: #cb151b !important;
}