How to reproduce:
1. Create a new Unity project
2. Download and import the ProBuilder package
3. Open Tools > ProBuilder > ProBuilder window and select 'Smoothing'
4. Toggle the 'Normals' button

Expected result: the state of the button is clearly visible when enabled
Actual result: the state of the button visually appears as 'disabled' even though it is enabled

Reproducible with: 2019.3.0a3, 2019.3.0b5, 2020.1.0a6
Not reproducible with: 2018.4.10f1, 2019.2.7f2, 2019.3.0a2
Could not test with: 2017.4.32f1(ProBuilder package unavailable)

Reproducible with package versions: 4.0.2, 4.1.0
Not reproducible with package versions: 3.10.1

Notes:
The issue is unaffected by the ProBuilder package version, only the Unity version