Warn the case when a string is being merged with an object

src/components/themr.js

@@ -183,6 +183,12 @@ export function themeable(original = {}, mixin) {
 
       let newValue
 
+      //when you are mixing an string with a object it should fail
+      invariant(!(typeof originalValue === 'string' && typeof mixinValue === 'object'),
+        `You are merging a string "${originalValue}" with an Object,` +
+        'Make sure you are passing the proper theme descriptors.'
+      )
+
       //check if values are nested objects
       if (typeof originalValue === 'object' && typeof mixinValue === 'object') {
         //go recursive

test/components/themr.spec.js

@@ -555,4 +555,10 @@ describe('themeable function', () => {
     const result = themeable(themeA, themeB)
     expect(result).toEqual(expected)
   })
+
+  it('throws an exception when its called mixing a string with an object', () => {
+    expect(() => {
+      themeable('fail', { test: { foo: 'baz' } })
+    }).toThrow(/sure you are passing the proper theme descriptors/)
+  })
 })