Vanilla CSS grid with a sprinkling of JS

I'm mostly happy with this, but getting the full width background sections while maintaining the grid was a challenge, and I don't think I found the best solution for that. I'll be interested to see how other people approached that.

Also, positioning the curvy background patterns, I'm 100% sure I didn't do that in the right way, but I couldn't figure out a better option for them.

Maybe instead of having a 12 column grid, within a margin, I should have tried a 14 column grid where the first and last columns are the main page margins.