You can customize the look and feel of almost every aspect of Splunk via CSS - if you are talking about existing UI elements. However, if you want to create your own kind of widget (a circular dashboard panel instead of a rectangle, perhap?), then that will require a great deal more work.
Before you consider extending the UI framework, I suggest that you take a look at the Sideview Utils app on Splunkbase: http://splunk-base.splunk.com/apps/36405/sideview-utils There is a free version and a paid version. This is a really powerful addition to Splunk UI development.