Followup 6a4b979514
In this commit I changed how we load the selected palette
stylesheet so the system specs would know when the CSS is actually
loaded, however this introduced an issue where the user would see
a FOUC when switching palettes. I also introduced a bug where
if you chose to force dark mode then switched the palette, you would
see light mode instead.
This commit fixes both issues, by swapping the CSS link with a preloaded
one then waiting for the preload before switching the href of the
existing link, and removes `mode` shenanigans causing the light mode
issue.
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.
This commit adds the above for these core site
settings:
* search_experience
* enable_welcome_banner
Since for horizon we want better control of
the default experience of these settings OOTB.
See also https://github.com/discourse/discourse/pull/31917
