Use theme tabRow color as acrylic tint (#19604) (#19622)

## Summary of the Pull Request
When `useAcrylicInTabRow` is enabled, the theme's `tabRow.background`
color is now used as the acrylic tint instead of being ignored.

## References and Relevant Issues
Fixes #19604

## Detailed Description of the Pull Request / Additional comments
Previously, the code had mutually exclusive branches for handling
`useAcrylicInTabRow` and theme `tabRow.background`. When acrylic was
enabled, the theme color was never evaluated.

This PR extracts the theme's tabRow color first, then uses it as the
acrylic tint if set, falling back to the default if no theme color is
specified.

## Validation Steps Performed
Tested with `useAcrylicInTabRow: true` and custom `tabRow.background` -
acrylic now shows the theme color as tint

## PR Checklist
Closes #19604

(cherry picked from commit 691568195b0bf2d392b3de2b0b1989111f3e082d)
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