The extension follows the theme color. You can find this option when you go to:
Form or Portal Settings
Styling & Customization section
Pick the theme color from the list of pre-defined options.
Optionally, turn on Set background color to theme color. This tints the background for the extension itself, and for the login page, if applicable.
Set the theme color after creating the extension, in Styling & Customization.
💡Child Form
Setting the theme color will affect the button field and the child form.
Related
After changing this setting, click Publish so your users see the change. Saving or previewing the draft does not update the published version.