Fix/typescript types (#26)

* fix Fix partial theme property

* fix Theme is an optional type

Author: Esemesek

typings/__tests__/index.test.tsx

@@ -55,5 +55,6 @@ const ThemedTitle = withTheme(TitleComponent);
 const App = () => (
   <ThemeProvider theme={themes.default}>
     <ThemedTitle title="React Theme Provider" theme={{ primaryColor: 'pink' }} />
+    <ThemedTitle title="Second title" />
   </ThemeProvider>
 );

typings/index.d.ts

@@ -11,7 +11,7 @@ export type ThemingType<Theme, PartialTheme> = {
   ThemeProvider: ThemeProviderType<Theme>;
   withTheme: <Props extends { theme: Theme }>(
     Comp: React.ComponentType<Props>
-  ) => React.ComponentType<Without<Props, "theme"> & { theme: PartialTheme }>;
+  ) => React.ComponentType<Without<Props, "theme"> & { theme?: PartialTheme }>;
 };
 
 // Library exports