You can add the calendar to any theme by adding this line into the sidebar:
<?php get_calendar(); ?>
Styling is a whole other thing, however. You’ll need to add this stuff into the css:
#wp-calendar {
border:1px solid #F1EEE9;
empty-cells:show;
font-size:85%;
margin:5% 0;
width:90%;
}
#wp-calendar #next a {
padding-right:10px;
text-align:right;
}
#wp-calendar #prev a {
padding-left:10px;
text-align:left;
}
#rmenu #wp-calendar a {
color:#2F3E4F;
display:block;
font-size:100%;
text-decoration:none;
}
#wp-calendar a:hover {
background:#E6E6E0;
color:#2F3E4F;
}
#wp-calendar caption {
color:#F1EEE9;
font-size:16px;
text-align:left;
}
#wp-calendar td {
color:#E6E6E0;
letter-spacing:normal;
padding:2px 0;
text-align:center;
}
#wp-calendar td.pad:hover {
background:#E6E6E0;
}
#wp-calendar #today {
background:#E6E6E0;
color:#2F3E4F;
}#wp-calendar th {
font-style:normal;
text-transform:capitalize;
}
And you may have to enclose the calendar call (the php line above) in <li></li>
tags, depending.
[Edit: the css stuff, colors etc., is specific to one of my own themes; you’ll need to change it to fit yours.)