Editing pages. Code font colour is white

I have a strange problem.

For the past few days I’m unable to edit any pages because the code text doesn’t show up until I highlight it. It’s basically white text on a white background.

I’ve disabled all my plugins one by one and also cleared out the cache numerous times but still it fails to appear.

Any clues please?

Hidden text: https://www.dropbox.com/s/p4ekjpnhq9vyrg4/Screenshot%202021-03-27%2012.10.31.png?dl=0

Highlighted text: https://www.dropbox.com/s/0ysygmzeq91r9q1/Screenshot%202021-03-27%2012.10.50.png?dl=0