So whenever an application is used, plank will change it's background to highlight it. Is there a way i can disable that? For an example when i start firefox there is that orange "shadow" behind it.
There appears to be no way to change the icon highlighting from within the Plank Settings or the dock.theme file itself. This could be something that is hard-coded and unchangeable.
But I would recommend copying one of the theme folders and playing around with the configuration in the dock.theme file.
You should be able to find your themes here... ~/.local/share/plank/themes or in your /usr/share/plank/themes directory.
You can hide it when it's covered by any window in the settings.
Settings -> Desktop -> Plank -> "Hide when..."