Implement ThemeProvider into Root core container

To provide the global theme (1), the `ThemeProvider` (2) component has
been implemented into the `Root` core container component (3). The
global theme, placed in `src/styles/themes` is passed to the provider to
make the theme globally available.

References:
  (1) https://www.styled-components.com/docs/advanced#theming
  (2) https://www.styled-components.com/docs/api#themeprovider
  (3) #36

Associated epics: GH-51
GH-53

src/components/containers/core/Root/Root.jsx

@@ -9,10 +9,10 @@
 
 import React, { Fragment } from "react";
 import PropTypes from "prop-types";
-import { createGlobalStyle } from "styled-components";
+import { ThemeProvider, createGlobalStyle } from "styled-components";
 import modernNormalize from "styled-modern-normalize";
 
-import { global, normalize } from "styles/theme";
+import theme, { global, normalize } from "styles/theme";
 
 /* eslint-disable import/extensions */
 import "inter-ui";
@@ -42,7 +42,7 @@ const GlobalStyle = createGlobalStyle`
 const Root = ({ children }) => (
   <Fragment>
     <GlobalStyle />
-    <Fragment>{children}</Fragment>
+    <ThemeProvider theme={theme}>{children}</ThemeProvider>
   </Fragment>
 );