fix: StepperSeparator should use background color

[meghprkh]

🔗 Linked issue

Similar to #1380

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The only other case I found in ui, where background was not set but foreground was used. The others are text-muted-foreground and bg-background text-foreground which should be good. Some blocks have this pattern too.

📸 Screenshots (if appropriate)

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.