Dark mode + light syntax highlighting leads to contrast problems for rich text suggestion diff editing

To reproduce:

  1. Set your user to use dark mode (dark or auto)
  2. Set your user to use Light syntax highlighting
  3. Remove any custom diff colors for your user
  4. Go to an MR
  5. Comment on a line in the diff
  6. Ensure you are using rich text editing
  7. Add a suggestion

You will see that the suggested addition in the suggestion diff while you edit has text that cannot be read against the diff addition background color

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