Bug 1703807 contains a screen of the spec how the in-content checkboxes should look like. The current implementation does not follow this spec. The mockup shows that there should be at least some spacing around the checkmark but in the current implementation there is not really any spacing around the checkmark, the checkmark sticks to the border of the checkbox and this is looking really bad.
I attached a second screenshot. It's far away from being a high quality comparison screen but the mockup was a low quality screen and I tried to show the checkbox in the mockup and the checkbox in the current implementation at the same size and with a large zoom to demonstrate the difference in spacing around the checkmark (top: mockup, bottom: implementation).