Rounded edges for boxes? (a la twitter widget)

So I have the code from twitter.com installed in my sidebar (as text) because I didn’t like any of the wp-specific twitter ones. The one thing it does is these rounded corners on the boxes. I’d love to use those on my whole site but have no idea if that’s doable in WordPress CSS or exactly where to put it… any thoughts?

The twitter widget code is:

<script src="https://widgets.twimg.com/j/2/widget.js"></script>
<script>
new TWTR.Widget({
  version: 2,
  type: 'profile',
  rpp: 4,
  interval: 6000,
  width: 180,
  height: 300,
  theme: {
    shell: {
      background: '#222222',
      color: '#ffffff'
    },
    tweets: {
      background: '#000000',
      color: '#ffffff',
      links: '#888888'
    }
  },
  features: {
    scrollbar: false,
    loop: false,
    live: false,
    hashtags: false,
    timestamp: true,
    avatars: true,
    behavior: 'all'
  }
}).render().setUser('twitterusername').start();
</script>