[@mantine/core] Stepper: fix separator color

[gdostie]

Hello 👋 I noticed that the Stepper separator had a different color between horizontal and vertical orientations. For instance you can see this on the docs website:

Vertical	Horizontal
mantine-color-gray-1	mantine-color-gray-2

While looking at the code I saw some dead code so I figured I would open a PR with a small cleanup. Here's a summary of what I did:

• getStyles('separator') is only every applied when orientation === 'horizontal' so the whole rule for .separator &:where([data-orientation='vertical']) can be deleted.
• defined --stepper-outline-color variable to make sure both orientations use the same color for the separator.
  • I chose the one that was previously applied for horizontal orientation (gray 2), let me know if that works for you.
  • I also apply this color to the non completed step background since it was the same for vertical, but I am unsure if it is ok (looks okay to me, but can revert if needed).
• defined a --stepper-outline-thickness variable for ease of customization

Here is a before and after

Before	After

[rtivital]

Thanks!