Can you style ordered list numbers?

I'm trying to style the numbers in a ordered list, I'd like to add background-color, border-radius and color so they match the design I'm working from:

I guess it's not possible and I'll have to use different images for each number i.e.

ol li:first-child {list-style-image:url('1.gif')};
ol li:nth-child(2) {list-style-image:url('2.gif');} 
etc...

Is there a simpler solution?

You can do this using CSS counters, in conjunction with the :before pseudo element:

 ol {
   list-style: none;
   counter-reset: item;
 }
 li {
   counter-increment: item;
   margin-bottom: 5px;
 }
 li:before {
   margin-right: 10px;
   content: counter(item);
   background: lightblue;
   border-radius: 100%;
   color: white;
   width: 1.2em;
   text-align: center;
   display: inline-block;
 }

<ol>
  <li>item</li>
  <li>item</li>
  <li>item</li>
  <li>item</li>
</ol>

I was looking for something different, and found this example at CodePen;

try this: http://codepen.io/sawmac/pen/txBhK

body {
  font-size: 1.2em;
  font-family: "Helvetica Neue", Helvetica, sans-serif;
  margin: 50px;
}
.custom-counter {
  margin: 0;
  padding: 0;
  list-style-type: none;
}
.custom-counter li {
  counter-increment: step-counter;
  margin-bottom: 5px;
}
.custom-counter li::before {
  content: counter(step-counter);
  margin-right: 20px;
  font-size: 80%;
  background-color: rgb(180, 180, 180);
  color: white;
  font-weight: bold;
  padding: 3px 8px;
  border-radius: 11px;
}

<ol class="custom-counter">
  <li>This is the first item</li>
  <li>This is the second item</li>
  <li>This is the third item</li>
  <li>This is the fourth item</li>
  <li>This is the fifth item</li>
  <li>This is the sixth item</li>
</ol>

2021 update, you can use the ::marker pseudo element selector in recent browsers: https://developer.mozilla.org/en-US/docs/Web/CSS/::marker

 li::marker {
   color: red;
 }
 li {
   color: blue;
 }

<ol>
  <li>item</li>
  <li>item</li>
  <li>item</li>
  <li>item</li>
</ol>