[AIM] Add dark mode support

This CL adds proper dark mode coloring to the composebox in the Lens
side panel. Since the composebox can be over the regular SRP, or the AIM
results, each of which with different color themes, the composebox needs to have different themes for each.

Luckily, many of the colors are shared between the two, so only some colors need to be overridden. Adds a mojo
communication to be able to respond to AIM results in the iframe.

Demo: http://shortn/_Y2RSPND944

Fixed: 439036490

Change-Id: I27ad668685866008f536c4435439591423b661b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6862002
Commit-Queue: Duncan Mercer <mercerd@google.com>
Reviewed-by: Juan Mojica <juanmojica@google.com>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503695}