Ide o pravekú chybu Wordu. Musíš to upraviť podľa tohoto: Background gradients print repeated or tiled. How to adjust?

I've encountered this as well and what I did is add a rectangle to the header/footer layer, resized it to fit the page, and filled it with a gradient.

To edit the header/footer: right-click in the header or footer area and then click Edit Header/Footer. You may want to decrease your zoom so you can see the entire page.

To insert a shape: Use the Insert tab, Shapes gallery, select the rectangle and draw it to fit the page.

To change the fill: On the Drawing Tools Format tab, in the Shape Style group, click Shape Fill, click gradient, and then click More Gradients. Modify the gradient style to your preference.