[core/themes] Add theme loading

Load a theme from a JSON file.

tests/test_theme.py

@@ -1,3 +1,25 @@
-# pylint: disable=C0103,C0111
+# pylint: disable=C0103,C0111,W0703
+
+import unittest
+from bumblebee.theme import Theme
+from bumblebee.error import ThemeLoadError
+
+class TestTheme(unittest.TestCase):
+    def setUp(self):
+        pass
+
+    def test_load_valid_theme(self):
+        try:
+            Theme("solarized-powerline")
+        except Exception as e:
+            self.fail(e)
+
+    def test_load_nonexistent_theme(self):
+        with self.assertRaises(ThemeLoadError):
+            Theme("no-such-theme")
+
+    def test_load_invalid_theme(self):
+        with self.assertRaises(ThemeLoadError):
+            Theme("invalid")
 
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