TypeError: Cannot read properties of undefined (reading 'dark')

[karstengresch]

Describe the bug
This error message is shown to the left of the Netlify CMS:

"
TypeError: Cannot read properties of undefined (reading 'dark') at /admin/cms.js:75:9481 at o (/admin/cms.js:2:837291) at R (/admin/cms.js:2:831433) at /admin/cms.js:2:832847 at /admin/cms.js:2:833025 at /admin/cms.js:2:11128 at of (/admin/react-dom.production.min.js:109:301) at ni (/admin/react-dom.production.min.js:123:490) at Rk (/admin/react-dom.production.min.js:254:168) at Si (/admin/react-dom.production.min.js:195:185)
"

To Reproduce
Open an existing article - the error message appears prior to any save or other action.

Expected behavior
No error message.

Screenshots

Applicable Versions:

• Netlify CMS version: netlify-cms-app@2.15.72
• Git provider: git-gateway
• Browser version: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36

CMS configuration

backend:
  name: git-gateway
  repo: openpracticelibrary/openpracticelibrary
  branch: main
publish_mode: editorial_workflow
media_folder: static/images
public_folder: /images
slug:
  encoding: ascii
  clean_accents: true
  sanitize_replacements: "-"
  sanitize_replacement: "-"
collections:
  - name: practice
    label: Practice
    publish: false
    delete: false
    folder: src/pages/practice
    create: true
    slug: "{{slug}}"
    fields:
      - label: Template Key
        name: templateKey
        widget: hidden
        default: practice-page
        required: true
      - label: Title of practice
        hint: The name of the practice
        name: title
        widget: string
      - label: Objective
        hint: One liner main objective for the practice
        name: subtitle
        widget: string
      - label: Publish Date
        name: date
        widget: datetime
      - label: Authors
        hint: Comma-separated list of case-sensitive github usernames eg., Springdo,
          rdebeasi, TdBeattie
        name: authors
        widget: list
        default: []
      - label: Tags
        hint: Where does this practice live on the Library Loop?
        name: tags
        widget: select
        multiple: true
        required: false
        max: 3
        options:
          - label: Validate (Discovery Loop)
            value: validate
          - label: Value (Discovery Loop)
            value: value
          - label: Culture (Foundation)
            value: culture
          - label: Methods (Foundation)
            value: methods
          - label: Ideate (Options Pivot)
            value: ideate
          - label: Insight (Options Pivot)
            value: insight
          - label: Measure (Delivery Loop)
            value: measure
          - label: Learn (Delivery Loop)
            value: learn
      - label: Mobius Loop Tag
        hint: Where does this practice live on the Mobius Loop?
        name: mobiusTag
        widget: select
        multiple: false
        required: true
        options:
          - label: Discovery
            value: discovery
          - label: Delivery
            value: delivery
          - label: Foundation
            value: foundation
          - label: Options
            value: options
      - label: Cover Image
        name: icon
        hint: Image to be used on the home screen and cards for the practice
        widget: image
        required: true
        default: https://github.com/openpracticelibrary/opl-media/blob/master/images/Needs%20an%20Image.png?raw=true
      - label: What is this practice?
        name: whatIs
        widget: markdown
      - label: Why do this practice?
        name: whyDo
        widget: markdown
      - label: How do I execute this practice?
        name: howTo
        widget: markdown
      - label: Media Gallery
        name: mediaGallery
        widget: list
        fields:
          - label: Media Link
            name: link
            widget: string
      - label: Links we love
        name: resources
        widget: list
        fields:
          - label: Link
            name: link
            widget: string
          - label: Link Type
            name: linkType
            widget: select
            options:
              - label: web
                value: web
              - label: purchase
                value: purchase
              - label: podcast
                value: podcast
              - label: video
                value: video
          - label: Description
            name: description
            widget: string
      - label: No. of people required
        hint: How many peop
...