Customizing theme

Customizing theme using https://developer.atlassian.com/cloud/confluence/theme/ but to Create a Confluence theme: https://developer.atlassian.com/cloud/confluence/create-a-confluence-theme/#create-an-empty-connect-theme. it says The Theme API was built for a UI that Confluence has recently moved away from, and doesn't support the new UI. Based on Theme API usage to date, we are evaluating whether the Theme API will support the new UI.

Suggestion
It will be really useful for users to be able to customize the theme. Right now it's not possible and the only option is to look for Apps in Atlassian Marketplace.