Move flags had an unreadable foreground color when in dark mode. We now

use the appropriate background and text colors here. They're defined by

the theme, which will give code themes control in the future to avoid

clashing colors.
The indentation colors have also been updated to be a bit more readable

in both light and dark modes. Just as with move flags, these are defined

by the theme. They have been recolored, so instead of a blue, we now use

a diff-based red/green. The red indicates where it moved from, and the

green where it moved to. This can help provide a bit of a better visual

aid on the move.
There's also a fix to the top border color of diff headers, giving them

a softer appearance and matching the bottom border color.

Tested indentation changes and move flags in both light and dark modes.
Checked contrast ratios to make sure they're WCAG AA-compliant.