A Smart Campaign keyword theme.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v26/common/criteria.proto)

| Fields ||
|---|---|
| ## `keyword_theme` Union field. Either a predefined keyword theme constant or free-form text may be specified. `keyword_theme` can be only one of the following: ||
| ### `free_form_keyword_theme` | `string` Free-form text to be matched to a Smart Campaign keyword theme constant on a best-effort basis. |
| ### `keyword_theme_constant` | `string` The resource name of a Smart Campaign keyword theme constant. `keywordThemeConstants/{keyword_theme_id}~{sub_keyword_theme_id}` |