Hacky alternative

You can edit VSCode built-in markdown.css stylesheet directly:

• Windows:
  C:\Program Files\Microsoft VS Code\resources\app\extensions\markdown-language-features\media\markdown.css

• macOS:
  /Applications/Visual Studio Code.app/Contents/Resources/app/extensions/markdown-language-features/media/markdown.css

• Linux:
  /usr/share/code/resources/app/extensions/markdown-language-features/media/markdown.css

Credits

• https://code.visualstudio.com/docs/languages/markdown#_using-your-own-css
• https://www.codearco.net/en/tutorials/tools-software/change-preview-style-markdown-vscode/
• https://github.com/raycon/vscode-markdown-style