You can style the default appearance for your buttons. You can set the default font, colour, border, size, and whether you want your buttons to have rounded corners. These settings will apply to the buttons that appear in the News Carousel widget, the Photo Slideshow widget, and the Button widget.  

Click Site Settings. Then Buttons, which appears under the Look and Feel menu.

You can now modify the default button settings and click Save.

These changes will apply to any images contained in the photo slideshow widget that display button overlays, and to the Read More button applied to any news carousel widget.