Mark-
What specific customization are you hoping to accomplish with custom CSS? We may change the DOM structure with future updates and thus have no short-term plans to officially support custom CSS. We are looking at further options around dashboard and tile customizations though, so it’s helpful to understand your functional goal of customizing the CSS to see if there are enhancements we can make to the core customization capabilities. 
That being said, some people have used third-party browser add-ons to customize the CSS and have come up with some pretty cool designs.
Source: Share your cool SharpTools project! - #40 by Tim, post 40