How to reproduce:
1. Create a new Unity project
2. In the Hierarchy, right-click -> 3D Object -> Terrain
3. In the Inspector, switch to the "Paint Trees" mode
4. Click Edit Trees -> Add Tree
5. Observe the error helpbox in the opened "Add Tree" window

Expected results: The helpbox matches the Editor styling
Actual results: The helpbox doesn't match the Editor styling

Reproducible with: 6000.0.58f1, 6000.2.5f1, 6000.3.0b2

Reproducible on: macOS Sequoia 15.6 (M1)
Not reproducible on: No other environment tested