FIX: Color palette switching when Horizon is not default (#95)

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.

spec/system/page_objects/components/user_color_palette_selector.rb

@@ -29,6 +29,10 @@ module PageObjects
         has_css?(".user-color-palette-selector[data-selected-color-palette-id='#{palette.id}']")
       end
 
+      def has_loaded_css?
+        has_css?(".user-color-palette-selector.user-color-palette-css-loaded")
+      end
+
       def has_tertiary_color?(palette)
         computed_color_hex =
           page.evaluate_script(