RectTransform gizmos color cannot be customized

Steps to reproduce:

1. Create a legacy Canvas, add some elements (images, buttons)
2. Make a prefab out of it
3. Enter Prefab stage, set Prefab's context to Gray

Actual results:  the white lines on the white background are hard to see/interact with

Expected results: it's still possible to easily manipulate the gizmos, colors can be customized if needed

Reproducible with versions: all
Tested on (OS): 

Notes:

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