This adds dark mode colors to the datetime/time since columns in

datagrids, making them easier on the eyes at night. It uses a darker

green-to-red set of transition colors for the cells.
There are two things to note here:


We do not use palette colors (nor have we before). While palettes are

   good for many UI elements, they don't always provide the range or

   tone of colors best used to communicate some visual elements. What

   we have in the palette today are not a good fit here.


We no longer have the alternating row variants for these colors.

   We normally distinguish between rows by making every other row just

   a tiny bit darker, and previously did this for these columns. This

   felt more muddy in dark mode, and arguably didn't really help in

   light mode. Especially since we were using shading to communicate

   age. We now just use one color for both even and odd rows.


The borders between these rows have been removed. Originally these

   were there because rows themselves had borders. When we removed row

   borders, we kept the age borders. This change might take some getting

   used to, but it provides a cleaner look overall.

Tested the colors in both light and dark mode, with a range of ages

(simulated in the screenshots on this review request by manually changing

the classes).