Update online documentation on theme

[mpmansell]

Online documentation doesn't make it clear that the theme (light/dark) needs to match the main editor theme. Starting the default install when you have a dark theme on the editor means that the node names are not visible and it is initially confusing. Less experienced users may not work out why this is the case and become frustrated.

[gregchamblin]

I have forked this project, and implemented several of the requests and fixes. Please check it out.

https://marketplace.visualstudio.com/items?itemName=GregChamblin.vscode-json-editor