White background flash on startup

### Expected
When launching, the background color should match the one of the (dark) theme.

### Observed
On launch, the program flashes with white background for less than a second.
No such issue appears on the web version.

### Reproduced

1. Launch Simplenote
2. Observe the flash

https://github.com/Automattic/simplenote-electron/assets/170368754/5276fc97-2e9e-46e1-9e72-18a409cf55e4

### Where did you see the bug
Linux Mint 21.3
Simplenote flatpak stable 2.21.0

Start by reproducing the launch on Linux Mint 21.3 with Simplenote flatpak stable 2.21.0 and compare startup behavior under the dark theme. Done means the application opens without a white background flash and the initial background matches the dark theme.

Written by the indexing model from the issue text.