[themes] high contrast theme not switched to regular theme on startup

[hediet]

Testing #145616

Steps:

1. Set a color theme in VS Code (no high contrast) -> Works
2. Enable high contrast theme in OS (Windows) -> VS Code switches to high contrast theme
3. Restart VS Code -> VS Code stays in high contrast theme
4. Disable high contrast theme in OS -> VS Code stays in high contrast theme!

For step 4, I would have expected that VS Code would go back to the normal color theme.
When leaving out step 3, it does so.

[aeschli]

Yes, I know it's inconsistent, but it's something we do on purpose: Never switch away from a high contrast theme on restart. IMO it's time to remove that. @bpasero any objections?

[bpasero]

No, feel free to change.

[aeschli]

This has been fixed in the meantime