This all happened in (I believe) the previous update. There is a discussion about it here: Talking about Markdown's CodeMirror theme update
You will need to add a little simple css to get things to be the colours you like. Best take a look at this thread (sorry, it's long): Share your CSS and maybe post a question there. But since it doesn't actually relate to this update, better to not discuss it here. You will totally be able to fix this, and probably make the editor even easier to read for you in the process. You may even find a new font that works better for you.