When the user add searx as a search engine, the browser loads the /opensearch.xml URL without the cookies.
Without the query parameters, the user preferences are ignored (method and autocomplete).
In addition, opensearch.xml is modified to support automatic updates,
see https://developer.mozilla.org/en-US/docs/Web/OpenSearch
Two colors are needed so I use a space in the cookie and a split in jinja to "encode" and "decode" them.
We should enforce that each theme if they must use a cookie, prefix its name with the name of the theme.
The color proposed here are based on bootstrap. We could use others.
