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Nothing about the dock could be changed, except for the general background color that is based off of the wallpaper that is used on the home screen.
 
It doesn't matter what wallpaper I use, the dock is flat gray and nothing can be seen through it. Not sure what you mean about background color.
 
It doesn't matter what wallpaper I use, the dock is flat gray and nothing can be seen through it. Not sure what you mean about background color.

Open Settings --> General --> Accessibility --> Increased Contrast. Is it On (green) or off? If it's on, turn it off. If it's off, turn it on and then back off.

Your dock should change colors based on your background. If none of that works, restart your device. Use different colored wallpapers - blue, green, red, etc.. See if it changes.
 
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