Fix the issue of difficult to see inline prediction clearly in Windows Terminal

[Golevka2001]

The origin theme works well in Powershell. But in Windows Terminal, the inline prediction nearly integrate with the background:

After adding "InlinePrediction" = [ConsoleColor]::DarkGray:

No changes to display in Powershell:

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Maybe the color DarkGray was not designed to display it, but I didn't find another color more suitable. And it actually did improve my reading experience.