You can choose to apply color themes to your diagram for quick and easy styling. Clicking the Diagram Themes button from the Toolbar will open up the Themes box on the right side of your diagram.

This moveable box can be moved anywhere on your canvas.

Hovering over each theme creates a preview of it applied to the diagram on your drawing canvas. Clicking on the theme applies it to your diagram. To undo an applied theme, select Edit->Undo, or Ctrl+Z.

Completed diagram with no theme:

Diagram Theme applied:

Please note that themes only work on flowchart and basic shape types.