Hi!
Yes, with the border options on the Theme panel. On the Theme Options -> Overall box layout panel and the Theme Options -> Input field layout panel, there is an option to adjust the border radius: http://i.imgur.com/c9qRX8c.png and http://i.imgur.com/QziTTKG.png
Try setting both to the same values, as some themes use the overall border, some only use the input border values.