Hi, I created a new custom theme and theme_fields.yaml. But {{ dump(this.theme.site_name) }} in my theme layout outputs NULL and {{ this.theme.site_name }} outputs nothing. Please help to fix this out.

I use this theme_fields.yaml: https://github.com/responsiv/clean-theme

===================================

#  Field Definitions
# ===================================

fields:
    site_name:
        label: Site name
        comment: The website name as it should appear on the front-end
        default: MY WEBSITE NAME
        span: auto

    site_motto:
        label: Site motto
        comment: A short statement about the website.
        default: Get back to basics
        span: auto

    facebook_url:
        label: Facebook URL
        span: auto
        default: https://www.facebook.com/

    twitter_url:
        label: Twitter URL
        span: auto
        default: https://www.twitter.com/

I forgot to add form: theme_fields.yaml into theme.yaml. Now it works perfectly.