This change updates the diff viewer to support dark mode. We use the

following palette:

Diff headers: Cool grey
Equals lines: Black background, dark grey line number background
Insert lines: Dark leafy green background, less-dark line number

  background
Replace lines: Dark tan background, light-tan line number background
Delete lines: Dark muted red, less-red line number background.
In-line changes: goldenrod background.

There are some structural changes to how we supply colors to the diff

viewer. Instead of several complex rules used to override colors and

prioritize borders, we now use CSS variables to define all our colors

for a <tbody>, letting us apply them in a standard way. Borders are

now alpha-transparent colors, helping to visually merge borders

together.
The colors are defined by the syntax highlighting theme. This will later

allow other diff themes to choose colors more appropriate for their

styles. We provide a default set. These colors are chosen based on

what both looks good and provides suitable contrast for all chunk types,

and is not tied to our standard color palette.
There's also a fix included here for showing new and deleted files

correctly in the mobile diff viewer.
At this time, binary file review does not have a dark mode appearance.

That will need to be addressed separately.

Tested a variety of changes in light and dark mode, mobile and desktop.
Tested highlighted lines and in-page diff navigation.
There will no doubt be further changes needed as we test more.