Steps to reproduce:

1) Open Unity and create a new project
2) Press Play button
Note: Play/Pause button colors will be not correct and will have a small white outline

Expected result: No white outline around those buttons (check attached image "expected.png")
Actual result: White outline around the buttons (check attached image "actual.png")

Reproduced with:
2018.2.0a8, 2018.2.0b1

Not reproduced with:
2018.1.0f1, 2018.2.0a7

Regression since:
2018.2.0a8

Note: not reproduced using Windows 10 machine