[theme] only restore the previous theme if the user has not changed the theme

[bpasero]

Testing #106389

• configure a HC default theme Abyss
• enter HC
• theme changes to Abyss
• manually change theme to Kimbie
• leave HC

=> 🐛 all my theme changes are lost

I think I would only restore the previous theme if the user has not changed the theme meanwhile?

[aeschli]

Fixed by the changes for https://code.visualstudio.com/updates/v1_89#_auto-detect-system-color-mode-improvements