When Horizon is not the default theme, and the color palette
switcher is used, we were ending up in a broken CSS state because
the color definitions were not loaded correctly when calling
`/color-scheme-stylesheet/` to get the new stylesheet URLs.
The problem was that we were not giving the theme ID to this URL,
we were only doing `/color-scheme-stylesheet/:palette_id`, not
`/color-scheme-stylesheet/:palette_id/:theme_id`, so the theme
color definitions like the ones that define the background color
were not loaded.
This commit fixes the issue and also changes to recreating the
`<link>` tags for the CSS in the head so we can check if they
are loaded correctly for easier testing in system specs.
