Add settings for a theme

Is it possible add a (global) settings form (blueprint) for a specific theme? I mean, when i change the background color for the whole theme instead in pages.
What is the most practice way to do this?
Trying adding the code below in /user/config/themes/my-theme.yaml

@extends:
    '@parent'

form:
    fields:
        content:

            fields:
                myfield:
                    type: text
                    label: My Field

But doesn't work.

It's the blueprints.yaml file that holds the theme/plugin configuration. See for example Antimatter: https://github.com/getgrav/grav-theme-antimatter/blob/develop/blueprints.yaml and the blueprints guide

Hi flaviocopes,
Oh okay. Ik try it. Tonight.

Hi, it's working.
Adding a colorpicker :-)
Thema Bootstrap 4 base WEBKUST