The base classes for review UIs have been updated to support dark mode.

On light mode, things look essentially as they did before, while on dark

mode we now adopt the review request background color and correct text

colors. Some old legacy decisions (like a very-subtly-different special

tab color) have been removed in the process.
The Image Review UI specifically has had a number of updates to the way

the UI is structured:


We now use Ink.MenuLabel for the zoom menu.


Instead of float and absolute-position hacks for the header area, we

  now use CSS grid to position things the way we want, keeping captions

  properly centered in the process.


The image modes now use the rb-c-tabs component, instead of

  reinventing their own design. This fits in better with the rest of the

  UI.


The diff mode tabs bar has been positioned in the header area, instead

  of below it. It also no longer has its own background color.


The content area now has a minimum height, large enough to display the

  zoom menu (which was being clipped for small images).


The image is now vertically-centered within the content area, instead

  of aligned to the top.


This isn't the big refactor of the review UI CSS structure that we've

wanted to do, but gets us closer to it, and allows us to remove some

really crufty code and workarounds that we had before.
We'll likely rewrite much of this for Review Board 8, when we revisit

the review UI architecture.

Unit tests pass.
Tested image review UIs in light and dark mode for:

Single-revision image file attachments
Multi-revision image file attachments
Diffed image file attachments
Single-revision images in the diff viewer
Diffed images in the diff viewer