Color Scheme: Differentiate parameter reference and parameter assignment

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## Steps to Reproduce

Please see here:
`user` (parameter name) and `user` (parameter value) are the same color (both white).

It would be convenient if we distinguish them with a different color.
So I tried to modify the color scheme


But unfortunately, it changed both at the same time


## Version info

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.12.13+hotfix.8, on Microsoft Windows [Version 10.0.18362.836], locale en-MU)

[!] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
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[√] Android Studio (version 3.6)
[√] VS Code (version 1.43.2)
[!] Connected device
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