Wrong background color using Open3DScene

**Checklist**
- [x] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [x] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [x] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for master branch).

**Describe the issue**
Using Open3DScene set_background seems to result in slightly wrong color.
Attempting to match the 1E1E1E color of visual studio code results in 0D0D0D, am I missing something?

**Steps to reproduce the bug**
```
import open3d as o3d

class ColorTest:
def __init__(self):
self.window = o3d.visualization.gui.Application.instance.create_window(
"ColorTest", 640, 480)
self.scene = o3d.visualization.gui.SceneWidget()
self.scene.scene = o3d.visualization.rendering.Open3DScene(
self.window.renderer)

# Following line is supposed to set background color to 1E1E1E,
# alpha is ignored.
self.scene.scene.set_background([30/255, 30/255, 30/255, 1])

# Show something on screen
self.scene.scene.show_axes(True)

# Setup camera towards origin
bbox = o3d.geometry.AxisAlignedBoundingBox([-1, -1, -1],
[1, 1, 1])
self.scene.setup_camera(60, bbox, [0, 0, 0])

self.window.add_child(self.scene)

def main():
o3d.visualization.gui.Application.instance.initialize()
ColorTest()
o3d.visualization.gui.Application.instance.run()

if __name__ == "__main__":
main()
```
**Error message**
None

**Expected behavior**
FEngine (64 bits) created at 0000022BA0741050 (threading is enabled)
FEngine resolved backend: OpenGL

Window appears with a background darker then expected (0D0D0D) with the axes shown.

**Open3D, Python and System information**
```
- Operating system: Windows 10
- Python version: Python 3.8
- Open3D version: 0.17.0
- System architecture: x86_64
- Is this a remote workstation?: no
- How did you install Open3D?: pip
```

**Additional information**
No response