Blackboard variable drag preview text is nearly invisible when using Editor Dark theme

Steps to reproduce:

1. Create a new project
2. Make sure the Editor theme is Dark
3. Import the attached "MySimpleGraph.cs" into the Project
4. Go to Assets > Create > Graph Toolkit Samples > Simple Graph
5. Open the created graph
6. Click the "+" button to add a new variable and drag it to Canvas
7. Observe the drag preview label text

Actual results: The drag-preview label text is nearly invisible against the canvas background

Expected results: The drag-preview label text is clearly readable against the canvas background.

Reproducible with versions: 6000.3.20f1, 6000.5.5f1, 6000.6.0b5, 6000.7.0a3

Tested on (OS): macOS 26.5

Notes:

• Contrast ratio for text and background is around 1.14:1 which breaks Unity Standard: 0209 (https://unityeditordesignsystem.unity.com/fundamentals/color-palette) and is a usability concern