Changing the menu background colour so all one colour and no white box around it

Hi all,
Newbie here!
I am trying to customise the background colour on twenty seventeen, I am getting there, finally.
But, when I change the background colour on the menu/submenu, it has like a white box around it (when I go to the inspect panel, it is the ‘padding’ section, if that makes sense). I CAN NOT find where to fix this, I’d like it to be a solid colour.
PLEASE HELP! (and, sorry for the HORRIBLE description, like I said, total newbie!)

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