Here slider is used to increase the thickness for the calendar's border and colorpicker is used for choosing the color for the calendar's border.
Here is the code,
<mx:application mx="http://www.adobe.com/2006/mxml" width="100%" height="100%" layout="vertical" verticalalign="middle" backgroundcolor="white">
<mx:applicationcontrolbar dock="true" paddingtop="100" paddingbottom="100">
<mx:hbox width="100%" height="100%" horizontalalign="center" paddingtop="10" paddingbottom="10" paddingleft="10" paddingright="10" borderthickness="5" bordercolor="black" borderstyle="solid">
<mx:form stylename="plain">
<mx:formitem label="Change the color:">
</mx:colorpicker>
<mx:formitem label="Change the border Thickness:">
<mx:hslider id="borderController" minimum="1" maximum="5" value="1" snapinterval="1" tickinterval="1" livedragging="true">
</mx:hslider>
</mx:formitem>
<mx:spacer width="50">
<mx:datechooser bordercolor="{borderColor.selectedColor}" borderthickness="{borderController.value}">
</mx:datechooser>
</mx:spacer>
</mx:formitem>
</mx:form>
</mx:hbox>
</mx:applicationcontrolbar>
</mx:application>
See the demo
Flex Examples - Calendar ( Datachooser) Border Thickness and Color Assigning