Widget sidebar 1 column

Hi!
I use One Press Plus theme, and I want to use the category-widget in my right sidebar.
My problem is, that the list of categorys is listed in two columns, when shown on a big screen.
You can check it out here. Its the widget called ‘Kategorier’ (danish for Category).
It looks great on a smaller screen, where the categorys are listed in one column. I prefer the same when shown on big screens.
Is it possible to do that?
Kind regards
Jan

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