Here's how that went:

- Right clicking on the page outside the boxes, ok no special contextual menu

- Went into the page settings (three dot menu) and try to find the option there

- Tried to create a new block, saw "container" and see that I can add a background color and resize it to full width. That's not exactly what I want but it's close enough

- Get rid of all the other blocks except container, now the second contextual menu to resize it is hidden at the top outside the viewport.

- Create a navbar just to add a little space and be able to click it :)

- Resize the box and add the background color (unfortunately for me, I can only choose presets of non-ugly colors)

Besides this, I really like how if you already use notion you'll feel right at home. :)

Edit: Just saw the "custom color" box at the end there. My bad.