FIX: Refactor to prevent themes affecting core stylesheets (#7029)

If a theme setting contained invalid SCSS, it would cause an error 500 on the site, with no way to recover. This commit stops loading theme settings in the core stylesheets, and instead only loads the color scheme variables. This change also makes `common/foundation/variables.scss` available to themes without an explicit import.

app/assets/stylesheets/common/foundation/variables.scss

@@ -64,7 +64,7 @@ $line-height-large: 1.4; // Normal or small text
 // These files don't actually exist. They're injected by Stylesheet::Compiler.
 // --------------------------------------------------
 
-@import "theme_variables";
+@import "theme_colors";
 @import "plugins_variables";
 @import "common/foundation/math";
 

app/models/theme_field.rb

@@ -261,7 +261,7 @@ class ThemeField < ActiveRecord::Base
   def ensure_scss_compiles!
     if ThemeField.scss_fields.include?(self.name)
       begin
-        Stylesheet::Compiler.compile("@import \"theme_variables\"; @import \"theme_field\";",
+        Stylesheet::Compiler.compile("@import \"common/foundation/variables\"; @import \"theme_variables\"; @import \"theme_field\";",
                                      "theme.scss",
                                      theme_field: self.value.dup,
                                      theme: self.theme

lib/stylesheet/compiler.rb

@@ -10,7 +10,7 @@ module Stylesheet
 
       if Importer.special_imports[asset.to_s]
         filename = "theme.scss"
-        file = "@import \"theme_variables\"; @import \"#{asset}\";"
+        file = "@import \"common/foundation/variables\"; @import \"theme_variables\"; @import \"#{asset}\";"
       else
         filename = "#{asset}.scss"
         path = "#{ASSET_ROOT}/#{filename}"

lib/stylesheet/importer.rb

@@ -33,12 +33,19 @@ module Stylesheet
       import_files(DiscoursePluginRegistry.sass_variables)
     end
 
-    register_import "theme_variables" do
+    register_import "theme_colors" do
       contents = ""
       colors = (@theme_id && theme.color_scheme) ? theme.color_scheme.resolved_colors : ColorScheme.base_colors
       colors.each do |n, hex|
         contents << "$#{n}: ##{hex} !default;\n"
       end
+      contents
+
+      Import.new("theme_colors.scss", source: contents)
+    end
+
+    register_import "theme_variables" do
+      contents = ""
 
       theme&.all_theme_variables&.each do |field|
         if field.type_id == ThemeField.types[:theme_upload_var]

spec/models/theme_spec.rb

@@ -290,7 +290,7 @@ HTML
       theme.reload
       expect(theme.theme_fields.find_by(name: :scss).error).to eq(nil)
 
-      scss, _map = Stylesheet::Compiler.compile('@import "theme_variables"; @import "desktop_theme"; ', "theme.scss", theme_id: theme.id)
+      scss, _map = Stylesheet::Compiler.compile('@import "common/foundation/variables"; @import "theme_variables"; @import "desktop_theme"; ', "theme.scss", theme_id: theme.id)
       expect(scss).to include(upload.url)
     end
   end