- create a new folder under `/sitecore/system/Settings/Forms/Meta Data/CSS Class Options` and call it `Form`
 - add new items for every color you want to use for the background colors under `Form` folder using `/sitecore/templates/System/Forms/Meta Data/CSS Class Option` template and for the value field use e.g. `form-blue-background`, 'form-grey-background`, etc.:
[![enter image description here][1]][1]

 - select `/sitecore/system/Settings/Forms/Field Types/Form` item and in `Css Class Autocomplete Options` field select `/sitecore/system/Settings/Forms/Meta Data/CSS Class Options/Form` folder created earlier:
[![enter image description here][2]][2]

 - now when your content authors start typing first name of the CSS class in `CSS Class` field of `Styling` section in `Forms Editor`, there will be auto-suggested option for all the available styles that you define:

[![enter image description here][3]][3]

Remember to add proper styling in your CSS files for all the background options.

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**EDIT**

In Sitecore 9.2 there is no option for CSS Class Options metadata, but your content authors can still add any CSS class to the `Styling` section in Form Editor:
[![enter image description here][4]][4]

There is no way to add auto-suggestion for the classes so you need to inform them about all the possible options for form backgrounds.

  [1]: https://i.sstatic.net/FNYfS.png
  [2]: https://i.sstatic.net/m14d7.png
  [3]: https://i.sstatic.net/wYsH7.png
  [4]: https://i.sstatic.net/eVgYA.png