Board Thread:Support Requests - Getting Technical/@comment-28398720-20161101165306/@comment-1757994-20161102043759

You can add  to every URL. It won't disable any theme, but it stops Common, Monobook, and Wikia css files from loading.

Alternatively, you could probably use personal JavaScript to remove site css. So it would load, then you'd unload it, instead of stopping it from loading.