[Stepper] Add slight transition

I’m not sure what the policy on css transitions is in this repo, but here’s a suggestion.

I noticed that color transitions on the StepIcon component are instantaneous, even though the entire component is quite animated and fluid. The StepIcon goes through several colors (default -> active -> completed), and I believe a simple transition: color 300ms ease; already adds quite a bit more style to it.

Transition variables to be determined by the theme of course. It could be the same as the transition of the Collapse?

Here’s the change that would need to be made:

    MuiStepIcon: {
      root: {
        transition: 'color 300ms ease',
      },
    },

[xtrixia]

Hi 👋 If no one work on this, I’d love to help on this one 😃

[oliviertassinari]

@lfp2 Sure 😃