Also update background_color in web app manifest on theme change

[Hypfer]

Proposed change

When installing HA as a PWA using Firefox, the navigation bar on Android currently remains bright white even when using a theme with a dark background. The notification bar however does successfully pick up the primary color of the theme.

This is because a backend theme change updates the theme_color but not the background_color property in the manifest.json. Thus, I just extended that to also update background_color.

Honestly this seemed a bit too easy so sorry in advance if I missed something blatantly obvious.

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[home-assistant]

Hey there @home-assistant/frontend, mind taking a look at this pull request as it has been labeled with an integration (frontend) you are listed as a code owner for? Thanks!

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[TheJulianJES]

home-assistant/frontend#20558 is in the latest beta (2024.5.0) and seems somewhat related.
To confirm, did you try to see if the issue still persists on that version?

[Hypfer]

I did not check the latest beta, however I'm certain that the issue still persists on that version, as home-assistant/frontend#20558 is about the theme_color (the accent color) whereas this PR is about the background_color