Apply more contrast between plain text files and the Material background

[tristan957]

This is barely readable:

[tristan957]

Could someone tell me the key I need to change in my settings so that I can fix this myself? I would love to submit a PR for this.

[waldyrious]

I don't know myself, but I suppose the information should be somewhere in this page: https://code.visualstudio.com/docs/getstarted/theme-color-reference.