Visual Studio Code allows you to use your own CSS in the Markdown preview tab.
Custom CSS for the Markdown preview can be implemented by using the "markdown.styles": [] setting in settings.json. The stylesheets can either be https URLs or relative paths to local files in the current workspace.
For example, to load a stylesheet called Style.css, we can update settings.json with the following line:
{
"markdown.styles": ["Style.css"]
}