Press "Edit Properties..." button to edit button edges width. By default, when you add new button, its edges widths are set to 5. But if you will change preview button size to 100x50 or bigger, you will see that some image elements look "fuzzy" (left image on the picture below). It happened because the play buttons border is not within the edge area.
Set both horizontal and vertical edges width to 10. You can see that the result looks much more precise now. Border is not resized because it is within the edge area now (right image on the picture above).
Click "Save theme" button to save theme changes. Click "Close" button.