Add classes to the workspace for themes

Fixes #3097

spec/theme-manager-spec.coffee

@@ -247,6 +247,21 @@ describe "ThemeManager", ->
           # from within the theme itself
           expect($(".editor").css("background-color")).toBe "rgb(0, 152, 255)"
 
+    describe "theme classes on the workspace", ->
+      it 'adds theme-* classes to the workspace for each active theme', ->
+        expect(atom.workspaceView).toHaveClass 'theme-atom-dark-ui'
+
+        themeManager.on 'reloaded', reloadHandler = jasmine.createSpy()
+        atom.config.set('core.themes', ['theme-with-ui-variables'])
+
+        waitsFor ->
+          reloadHandler.callCount > 0
+
+        runs ->
+          # `theme-` twice as it prefixes the name with `theme-`
+          expect(atom.workspaceView).toHaveClass 'theme-theme-with-ui-variables'
+          expect(atom.workspaceView).not.toHaveClass 'theme-atom-dark-ui'
+
   describe "when the user stylesheet changes", ->
     it "reloads it", ->
       [stylesheetRemovedHandler, stylesheetAddedHandler, stylesheetsChangedHandler] = []

src/theme-manager.coffee

@@ -94,6 +94,7 @@ class ThemeManager
           console.warn("Failed to activate theme '#{themeName}' because it isn't installed.")
 
       Q.all(promises).then =>
+        @addActiveThemeClasses()
         @refreshLessCache() # Update cache again now that @getActiveThemes() is populated
         @loadUserStylesheet()
         @reloadBaseStylesheets()
@@ -103,10 +104,21 @@ class ThemeManager
     deferred.promise
 
   deactivateThemes: ->
+    @removeActiveThemeClasses()
     @unwatchUserStylesheet()
     @packageManager.deactivatePackage(pack.name) for pack in @getActiveThemes()
     null
 
+  addActiveThemeClasses: ->
+    for pack in @getActiveThemes()
+      atom.workspaceView?[0]?.classList.add("theme-#{pack.name}")
+    return
+
+  removeActiveThemeClasses: ->
+    for pack in @getActiveThemes()
+      atom.workspaceView?[0]?.classList.remove("theme-#{pack.name}")
+    return
+
   refreshLessCache: ->
     @lessCache?.setImportPaths(@getImportPaths())