Prerequisites:
You'll need Pro version of Unity Editor.

Steps to repro:

1. Open Unity Editor on OSX;
2. Create new project;
3. Import 'Corridor Lighting Example' package from Asset Store - https://www.assetstore.unity3d.com/en/#!/content/33630;
4. In API Updater popup window click 'I Made a Backup. Go Ahead!' button;
5. Wait until package is imported;
6. Observe Editor layout.

Expected result:
Editor coloring is stable and consistent (dark grey for Pro version). See attached expected.png screenshot.

Actual result:
Editor control windows are cirrupted - they are colored in light grey making UI elements hardly unreadable. See attached actual.png screenshot.
Regression introduced in Unity 2018.1.0a2.

Workaround is to re-launch Editor with the same project.

Notes:
- Reproducible in 2018.1.0a3, 2018.1.0a2;
- Not reproducible in 2018.1.0a1, 2017.2.0p1, 2017.1.0p4;
- Reproducible on OSX 10.12.6; not reproducible on Windows 10;
- Also reproducible when downloading projects from Collab.