The Format Painter is a tool in Axure RP that allows you to quickly copy and paste style properties from one widget to another. It's a "modeless" window (the opposite of "modal"), meaning you can have it open while simultaneously working on the canvas. It's super useful and has been part of Axure RP since forever, but it's one of those features that can kind of stay under the radar if you don't know to go looking for it.
As of Axure RP 8, you can access the Format Painter via the "More" dropdown in the Tools section of the main toolbar. (In previous versions it was available via a small icon in the Style toolbar.) Here it is in action:
You'll notice that the Format Painter dialog has a line item for each style property available in Axure RP, and each one can be checked or unchecked to include or exclude