apply diagram background styles, fix bounding box bugs

[alixander]

Also

• makes the bounding box of diagrams more accurate on the right/bottom sides.

(case of this having too much padding, which is now reduced)

(case of this having too little padding, which is now increased)

• brings back sketch test SVGs. not comparing their bytes, but good to have for visual comparisons

These are all with padding=0. Here's with some padding 10):

i've verified every e2e test delta

closes #663