Better theme for rendering Markdown

[nicoespeon]

We're currently using the Light+ theme from VS Code, which is great because it's natively installed & good enough.

I'd like to add some tweaks to make the raw Markdown even more readable:

• [ ] do not put ** chars in bold, to focus on the content and not the syntax

Useful resources

• https://github.com/microsoft/vscode/blob/master/extensions/theme-defaults/themes/light_vs.json
• https://css-tricks.com/creating-a-vs-code-theme/
• https://code.visualstudio.com/api/extension-guides/color-theme#create-a-new-color-theme