Revert "Apply the background color all the way ...
... to the right" and "Only create a CSS grid on code inside diffs". This reverts commit 55d02ddc and commit 7d564eb9. Creating a work around with CSS grid to apply a background color all the way to the right breaks diffs with more than 1000 lines, both in Chrome and Chromium. And breaks diffs with more than 10000 lines in Firefox. This solution could be used for small diffs, but not for all of them.