You can do one of two things.
1) The first is the simplest, just change the theme of the chart to metro as demonstrated here in the example application
. Notice in the left pane under the description that you can change the whole theme of that chart by assigning one of 9 built in telerik themes. This document
will show you several different ways you can set the theme for a RadChart. The same technique can be used for any of the RadControls for Silverlight.
2) The second thing you can do, which is more difficult and time consuming, is to create a custom style for the chart. This document
will explain there is a few different ways you can modify a theme, or create your own style from scratch. If you only want to change the bars, then you are looking to modify the Series style and this document
will show you how to do that.
I hope this helps you, there are many ways to alter the appearance of any of the RadControls. The theming mechanism is the quickest and is my personal favorite.