add createTheme helper

Author: steven-mercatante

src/themes/roli.js

@@ -1,7 +1,7 @@
-import merge from 'lodash.merge';
 import defaultTheme from './default';
+import { createTheme } from '../utils/themes';
 
-const roliTheme = merge({}, defaultTheme, {
+const roliTheme = createTheme(defaultTheme, {
   timeline: {
     backgroundColor: '#0C1734',
     a: { color: 'yellow' },

src/utils/themes.js

@@ -0,0 +1,5 @@
+import merge from 'lodash.merge';
+
+export function createTheme(baseTheme, newTheme) {
+  return merge({}, baseTheme, newTheme);
+}

src/utils/themes.test.js

@@ -0,0 +1,19 @@
+import { createTheme } from './themes';
+
+test('createTheme creates a theme', () => {
+  const baseTheme = { timelinex: { backgroundColor: '#fff' } };
+  const newTheme = createTheme(baseTheme, {
+    timelineTrack: { backgroundColor: '#000' },
+  });
+
+  const expected = {
+    timeline: {
+      backgroundColor: '#fff',
+    },
+    timelineTrack: {
+      backgroundColor: '#000',
+    },
+  };
+
+  expect(newTheme).toEqual(expected);
+});