Feature: Support for theme packs

### What feature or improvement do you think would benefit Files?

We are going to support for displaying a background image. It would make the distribution of themes for Files easier if the theme could be changed by double-clicking on a theme pack file, which contains a background image and other theme settings.

### Requirements

- Theme pack files have `.filestheme` extension.
- A theme pack file is actually a zip archive, which contains an image file for the background and a json file which stores theme settings.
- If the user opens a theme pack file, Files applies the background image and other theme settings contained within it.

### Files Version

3.3.6.0

### Windows Version

10.0.22631.3447

### Comments

_No response_

No files, tests, or entry points are named. Start by locating the existing theme and background-image handling and the path used when a user opens a file, then review how a .filestheme ZIP's image and JSON settings should be applied. Done means opening a theme pack applies both the background and its other settings.

Written by the indexing model from the issue text.