A new option called Apply theme to asset group has been spotted in Performance Max.
Advertisers now see three options when creating an asset group: New Asset Group, Copy Existing Asset Group, and the new theme application.

The themes focus on seasonal and cultural moments such as Black Friday, Christmas, and Mother’s Day. By applying a theme, Google assists in constructing relevant assets, saving time for advertisers who skip building seasonal groups from scratch. However, advertisers are advised to ensure that these themes align with their branding and offerings before utilization.
This update was discovered by Thomas Eccel.