Stepper styles and passthrough improvements

[sandrocsimas]

• If we set a background color different than white on a page, this is how the stepper header looks. There's a white background on the button. Instead of doing this, could you not set a background color, or define a padding for the button?
  Current style:

Without background color:

With padding:

• The panel has no border-radius. It could be like a card;

Current style:

With card border-radius:

• There's no way to customize stepper header and action through passthrough. These are all the options:

• There's no way to customize stepper globally:

[melloware]

PR is welcome