Match system theme does not always update when the system theme changes (macOS at least)

### Description

On the Element desktop app in macOS Catalina the theme is not getting updated from dark to light when the system theme changes (automatically or otherwise) despite the "match system theme" option being on. However, from light to dark the change seems to happen correctly.

### Steps to reproduce

- Turn on the "Match system theme" option
- Quit Element
- Go to macOS system preferences and set appearance to light (under general)
- Open Element
- Element has the light theme as expected
- Go back to macOS settings and set appearance to dark
- Element updates the theme to dark correctly
- Go back to macOS settings and set appearance to light
- Element does not update the theme back to light

### Version information

- **Platform**: desktop
- **OS**: macOS
- **Version**: 1.7.9