How can I round the corner of the widget?

To apply rounded corners first enter edit mode and click on the pencil icon. Next, click on the style tab. Then, click on the corners tool. Finally select the corners you want and change the radius.

Step 5: Select the corners (C).
Step 6: Set the radius of the corners you require to any size (D). You can use the standard slider or choose to override the pixel size (see screen shot below).
Before and after images:
Note: You can also round the corners of an image in design mode.