Font size inconsistent when moving GUI window between multiple monitors

When moving a GUI window from one monitor to another, the font is not consistent. The font size, relative to the window title font changes. Widget size (eg buttons / panels) also changes.

**To Reproduce**
Steps to reproduce the behavior:

1. Open the Open3D app. Note font size of the controls widgets
2. Move app window to external monitor. Font size changes.

**Expected behavior**
Relative font size (e.g. w.r.t window title font size) should be the same on all monitors

**Screenshots**
In laptop display:

GUI widget font is the same size as the window title font.

When the window is moved to the external monitor, we have:
GUI widgets font is now larger than the window title font. Other widgets are larger too.

**Environment (please complete the following information):**

- Operating system: OSX 10.15
- Python version: Any
- Open3D version: 0.11, 0.12
- Is this remote workstation?: No
- How did you install Open3D?: Download App from Github / Python / Build from C++ source
- Compiler version (if built from source): clang 12