“+” button below Project tab is barely visible when using Light Editor theme

Steps to reproduce:
1. Create new Unity project
2. Set Editor Theme to Light (Preferences > Editor Theme)
3. Observe the “+” button below “Project” tab

Actual results: “+” button is barely visible when Editor theme is set to Light
Expected results: “+” button is visible when Editor theme is set to Light

Reproducible with versions: 6000.5.0a5 (9b63d0bc55db)
Not reproducible with versions: 6000.5.0a4 (35ed6e8537f8)

Tested on (OS): Ubuntu 24.04, macOS Silicon Tahoe 26.2 (M1)