Separation lines are missing in Render Graph Viever window between Access Blocks in Light theme

Steps to reproduce:
1. Create new Unity project
2. Set Editor theme to Light
3. Open Render Graph Viewer window (Window > Analysis > Render Graph View)
4. Observe the window

Actual results: The separation lines between access block icons are missing or invisible
Expected results: The separation lines in the main window should be visible in Light Theme

Reproducible with versions: 6000.2.0a4, 6000.2.0f1, 6000.3.0a4

Not reproducible with versions: 6000.1.15f1, 6000.2.0a3

Tested on (OS): macOS Silicon, Sequoia 15.5 (M1)