Attach (recommended) or Link to PDF file here:
[Erstattung von Arbeitgeberaufwendungen nach __ 56 Abs. 1 und 57, 58 Infektionsschutzgesetz - Antrag(1).pdf](https://github.com/mozilla/pdf.js/files/6009589/Erstattung.von.Arbeitgeberaufwendungen.nach.__.56.Abs.1.und.57.58.Infektionsschutzgesetz.-.Antrag.1.pdf)

Also available at: [formularserver.bayern.de](https://formularserver.bayern.de/intelliform/forms/stmi+regierungen/rof/b5/55.2/rof_55.2-086/index)

Configuration:
- Web browser and its version: FF 85.0.2 and FF 86.0b9
- Operating system and its version: Win 10
- PDF.js version: 
FF 85.0.2:
const pdfjsVersion = '2.7.345';
const pdfjsBuild = 'b194c820b';

FF 86.0b9:
const pdfjsVersion = '2.7.510';
const pdfjsBuild = '631bada0d';

- Is a browser extension:
no
Steps to reproduce the problem:
1. load pdf document

What is the expected behavior? (add screenshot)
This is with latest Chrome:
![grafik](https://user-images.githubusercontent.com/584660/108499622-e2360880-72ae-11eb-82a7-b181619ff079.png)


What went wrong? (add screenshot)
The checkboxes for e.g. "Geschlecht" are black and you cannot see which one is checked. But this is true for every checkbox in that document.
![grafik](https://user-images.githubusercontent.com/584660/108499753-14e00100-72af-11eb-89d3-1f5ca652e058.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

const TEXT_ALIGNMENT = ["left", "center", "right"];
    const { fontSize, fontColor } = this.data.defaultAppearanceData;
    const style = element.style;
    
    
    
    if (fontSize) {
      style.fontSize = `${fontSize}px`;
    }
    style.color = Util.makeHexColor(fontColor[0], fontColor[1], fontColor[2]);
    if (this.data.textAlignment !== null) {
      style.textAlign = TEXT_ALIGNMENT[this.data.textAlignment];
    }
  }
}
class CheckboxWidgetAnnotationElement extends WidgetAnnotationElement {
  constructor(parameters) {
    super(parameters, { isRenderable: parameters.renderForms });
  }
  render() {
    const storage = this.annotationStorage;
    const data = this.data;
    const id = data.id;
    let value = storage.getValue(id, {
      value: data.exportValue === data.fieldValue,
    }).value;
    if (typeof value === "string") {