Generally the appearance of the RadButton (ButtonType="LinkButton") is different than the appearance of the RadButton (ButtonType="StandardButton"). The link buttons uses CSS3 while the standard button uses sprite image for its background image. There is, however, similarity between the appearance of both buttons in the described case - standard button with Metro skins look almost similar to the link button. Note that this is a sequence of the Metro skin in general.
If you want to approach the link button's appearance to the one of the standard button's you can apply rounded corners to it. For example:
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