FIX: Do not include theme variables in plugin SCSS, and fix register_css

spec/components/stylesheet/compiler_spec.rb

@@ -15,6 +15,41 @@ describe Stylesheet::Compiler do
     end
   end
 
+  context "with a theme" do
+    let!(:theme) { Fabricate(:theme) }
+    let!(:upload) { Fabricate(:upload) }
+    let!(:upload_theme_field) { ThemeField.create!(theme: theme, target_id: 0, name: "primary", upload: upload, value: "", type_id: ThemeField.types[:theme_upload_var]) }
+    let!(:stylesheet_theme_field) { ThemeField.create!(theme: theme, target_id: 0, name: "scss", value: "body { background: $primary }", type_id: ThemeField.types[:scss]) }
+    before { stylesheet_theme_field.save! }
+
+    it "theme stylesheet should be able to access theme asset variables" do
+      css, _map = Stylesheet::Compiler.compile_asset("desktop_theme", theme_id: theme.id)
+      expect(css).to include(upload.url)
+    end
+
+    context "with a plugin" do
+      before do
+        plugin = Plugin::Instance.new
+        plugin.path = "#{Rails.root}/spec/fixtures/plugins/my_plugin/plugin.rb"
+        plugin.register_css "body { background: $primary }"
+        Discourse.plugins << plugin
+        plugin.activate!
+        Stylesheet::Importer.register_imports!
+      end
+
+      after do
+        Discourse.plugins.pop
+        Stylesheet::Importer.register_imports!
+      end
+
+      it "does not include theme variables in plugins" do
+        css, _map = Stylesheet::Compiler.compile_asset("my_plugin", theme_id: theme.id)
+        expect(css).not_to include(upload.url)
+        expect(css).to include("background:")
+      end
+    end
+  end
+
   it "supports asset-url" do
     css, _map = Stylesheet::Compiler.compile(".body{background-image: asset-url('/images/favicons/github.png');}", "test.scss")