refactor(stepper): scope styles to component

[desig9stein]

Closes #

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• Bug fix
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[adrianptrv]

I found that some parameters from the stepper theming function don't work.

• $current-subtitle-color, $title-hover-color, $subtitle-hover-color, $current-step-focus-background and $invalid-step-focus-background - do not work when applied with the theming function, only their vars get declacred.

Out of these 5 parameters, only - $title-hover-color and $subtitle-hover-color do not work on the master branch either, for which I will raise a separate issue.

---

• $invalid-step-background and $current-step-background parameters also do not work as expected, they paint both the step and the step separator next to it with the set color."
  
  The issue is only seen here, on the master branch, they work fine:
  

It seems that the colors from these parameters are applied to the igx-step element, which contains the div for the step and the ::before pseudo-element that represents the step separator, which is also colored. On the master branch, the colors are applied only to the div that holds the step.

[adrianptrv]

I also found some discrepancies between some themes styles and their UI Kits.

On the dark version of the Fluent theme - the completed version of the step has a dark gray background, while in the UI kit, it has a light gray background.
UI Kit:

Current:

---

On the dark version of the Bootstrap theme - again the completed version of the step have a gray background and black icon/text color, while in the UI kit is with white icon/text color.
UI Kit:

Current:

---

On the dark version of the Material theme - the normal state of the step, when the stepper is not linear, has a gray background with black icon/text color, while in the UI kit, it has white icon/text color.
Ui Kit:

Current:

---

On all themes and versions, the progress bar (step separator) in vertical view doesn't get colored, while the one in horizontal view is colored.

[desig9stein]

@adrianptrv All comments, except the first and last, are a result of testing with an incorrect local schema. For instance, if your global theme is set to Fluent, but your local theme schema defaults to Material (default), inconsistencies will arise.

When creating a custom theme, ensure the schema matches the global theme—even if the global theme is Material Dark, the local schema will default to Material Light unless explicitly specified. Failing to do so can lead to various issues.

All other issues from the comments are fixed and can be tested