rectangularRadioButtonGroup should have a thicker stroke as default styling

[amanda-phet]

When discussing phetsims/mean-share-and-balance#210 we noticed that the styling for rectangularRadioButtonGroup is all over the place in sims. Sometimes the selected button has a black stroke, sometimes it is thicker, sometimes it is black and another color, sometimes it is just another color. The designers agreed that we should always ensure the selected button has a thicker stroke in addition to a color change, so that color alone is not being used for contrast.

Can we change the default behavior for this so that the selected option is a thicker stroke? This shouldn't be something we need to request, it should be standard.