Currently, there is no proper documentation on the various java scripts, css files etc that come along with the styles in BI EE. So, if you are part of the UI team and would like to customize OBI EE but do not know where to start, do not despair. There is a wonderful little extension called Firebug(only for firefox), that will let you know which css, html, java scripts are used where in the dashboards, answers etc. This extension is available only for firefox. If you want a similar one for IE, you can get the instructions of using firebug on IE here. You can download the Firebug extension for Firefox here
So, if you want to know which part of the screen a particular CSS is being used, just scroll over the CSS and that part of the screen will be highlighted as shown below. This utility is very handy if you want to make some minor modifications to your CSS files.
Just thought i would blog about it since this is a very common question on the customization front.