Hi,
a. Write a new css called "dashboard-extended.css" and put the following snippet inside it
@import 'dashboard.css'
#dashboard{
background-color:;
}
b. copy this file to your SPLUNK_HOME/etc/apps/search/appserver/static folder.
c. Go to your browser and hit the url http://: /en-US/debug/refresh. Click the refresh button.
d. Go to dashboard page and do a Ctrl+F5 ,for full reload . You should see the changes.
It's a two-step process - you have to first create a CSS file for your custom appearance, then reference that CSS file in the XML of your dashboard. Here are some useful links which will help you tie it together and also get information on other customization options
Customizing dashboards and custom stylesheets: http://docs.splunk.com/Documentation/Splunk/6.1.3/Viz/CustomizeSimpleXML#Custom_stylesheet
Simple XML reference: http://docs.splunk.com/Documentation/Splunk/6.1.3/Viz/PanelreferenceforSimplifiedXML#dashboard
Dashboard examples app: http://apps.splunk.com/app/1603/ - this is a great resource for any customizations you may want to do.