• VSCode Version: 1.49.0
• OS Version: Windows 10 2004

Steps to Reproduce:

1. Change VS Code to the built-in Quiet Light theme
2. Save the text below as test.diff and open it with VS Code

--- a.txt	2020-09-14 19:23:42.699158000 -0600
+++ b.txt	2020-09-14 19:23:41.501314200 -0600
@@ -1 +1 @@
-test
+hello world

3. Observe that with Quiet Light theme, syntax highlighting is not working:
   

Does this issue occur when all extensions are disabled?: Yes

Tested with all extensions disabled, also with the insider build, still have this issue. Note that with other themes, the syntax highlighting is working. Looks like this is an issue with the Quiet Light theme.