Now using Lightning Design System in Lightning Application is easier – No static resource is required to include CSS files. An application automatically gets Lightning Design System styles and design tokens if it extends force:slds.

[gist id=0af3738aecb9e7f87b4a7d5779bbe4f5 file=main.app]

Same behaviour can be accomplished in Visualforce pages with the help of apex:slds component. Include  /> in a Visualforce page to use Lightning Design System stylesheets in the page.

[gist id=0af3738aecb9e7f87b4a7d5779bbe4f5 file=main.page]