Ling Zhou, we do have different Style Definitions that come bundled with ThingWorx. We don't have Mashup themes per se, because every user's Mashup is different because it is styled and built based on their specification. So providing a specific theme wouldn't work out.
Well, judging by the image, I see there's a collapsible Menu on the left (with custom icons) and custom widgets to the right, with custom icons as well. By theme, the only thing that you can quickly replicate is the color scheme, with the title bar and the menu.
Style Definitions do have background, foreground and line colors to choose from. For example, Menus do have about 6-7 different styles for base colors, for actions etc. It would vary based on the widget being used.