Log warning when enabled theme isn't installed

spec/theme-manager-spec.coffee

@@ -224,3 +224,14 @@ describe "ThemeManager", ->
 
       runs ->
         expect($(document.body).css('border-style')).toBe 'none'
+
+  describe "when a non-existent theme is present in the config", ->
+    it "logs a warning but does not throw an exception (regression)", ->
+      waitsForPromise ->
+        themeManager.activateThemes()
+
+      runs ->
+        spyOn(console, 'warn')
+        expect(-> atom.config.set('core.themes', ['atom-light-ui', 'theme-really-does-not-exist'])).not.toThrow()
+        expect(console.warn.callCount).toBe 1
+        expect(console.warn.argsForCall[0][0].length).toBeGreaterThan 0

src/theme-manager.coffee

@@ -61,7 +61,14 @@ class ThemeManager
       @deactivateThemes()
 
       @refreshLessCache() # Update cache for packages in core.themes config
-      promises = @getEnabledThemeNames().map (themeName) => @packageManager.activatePackage(themeName)
+
+      promises = []
+      for themeName in @getEnabledThemeNames()
+        if @packageManager.resolvePackagePath(themeName)
+          promises.push(@packageManager.activatePackage(themeName))
+        else
+          console.warn("Failed to activate theme '#{themeName}' because it isn't installed.")
+
       Q.all(promises).then =>
         @refreshLessCache() # Update cache again now that @getActiveThemes() is populated
         @loadUserStylesheet()