Pulse allows custom css to be specified in style.css as shown on the help page:Custom Theme.

Every application and widget has a className and id that can be used for custom styling.

Name
#dashid-4 Gives a unique id for one entire application. It's based on a unique database ID assigned when a application is first created and never changes.
.dashname-Candlestick A unique class name for one entire application based on the name of the application. Note this may change if the author changes the name.
#widget-b8b07240-dee0-432b-9a41-eb65d8c0a68f Unique id for one wiget. i.e. One panel on a application.
.widget-candle A class name set for a given widget based on the title of the tab. This name can be changed.

The easiest way to find the widgetname-id is to use web developer tools.

Application CSS id and class names. Widget CSS id and class names.