Add theme files for grid widgets

[smhigley]

Type: bug Type: feature

The following has been addressed in the PR:

• [x] There is a related issue
• [x] All code has been formatted with prettier as per the readme code style guidelines

Description:

Resolves #27

Partially dependent on vdom changes in https://github.com/dojo/widgets/pull/600

[agubler]

Do you mind adding an image of the theme to the PR?