Improve difference in color between alignment column and straight double quotes

[tangledhelix]

Mentioned by @srjfoo in #544,

At least on default/dark, the highlight color for straight double quotes is either the same as the color for the column alignment highlighting, or very close. (I'd take the latter a bit darker, maybe?) Again, it may not be worth worrying about, but I thought I'd mention it.

Re-evaluate these two colors in dark mode and disambiguate them better.

[tangledhelix]

Relates to #534