Dark mode

Closes #61 and as a side effect, e/backlog#1266 (closed)

The current dark mode looks like:

Test environment

1. To make cache less problematic make sure all your volumes are deleted. You may wish to run (⚠ It will delete every container, image and volumes on your machine):

docker stop $(docker ps -aq) && \
docker rm $(docker ps -aq) && \
docker volume rm $(docker volume ls -q)
docker system prune -a

Then run

docker-compose up -d
docker-compose rm -sf spot
docker run -it --rm -v $(pwd):/ws -w /ws --hostname spot --network=my-spot_default --env-file .env registry.gitlab.e.foundation:5000/e/cloud/my-spot/env bash
PYTHONPATH=$(pwd) SEARX_DEBUG=1 python -X dev searx/webapp.py

Test that

1. You can change the etheme style to dark/light with the nav switch
2. That your current style is consistent even after
   1. Refreshing the page
   2. Going to the preferences page
   3. After making some search
3. Assert that the dark mode also works as expected on mobile device screen size
4. Assert that the dark theme looks good enough to be deployed

Still needs to be done:

• Dark mode styles
• Add dark mode as a new style for /e/ theme
• Switch button on top of home page when on /e/ theme
  • Save Switch state on user cookies/preferences