Official guideline for creating ImageGlass 9 theme pack

Hi all,

### The official guideline: https://imageglass.org/docs/theme-pack
### Example of `igtheme.json` file: [dark/Green-Gradient.Duong-Dieu-Phap/igtheme.json](https://github.com/ImageGlass/Theme/blob/main/themes/dark/Green-Gradient.Duong-Dieu-Phap/igtheme.json)

Visit https://imageglass.org/themes to download the theme packs that support v9.0.


Thank you!