note for all CSS comments: some of this stuff might just have been copied and pasted, but because our review tools are not great and I am all about quality, I will call out anything I see.

For 'top' and 'height' you used em instead of Rem.  If this was intentional, it should have a comment.

for the 100% - 9.5rem here, I'd love if we could have a comment explaining why that value is chosen.  Based on my understanding of the ingredients, I'd suggest:
/* Our goal is to leave the bottom toolbar visible, so 5rem for the header and 4.5rem for the bottom toolbar. */