Add a theme switcher component

At the moment we are just using default white theme coming from Material-UI for react. We should have a switcher component visible on the screen that allows us to toggle to the dark theme as well as any custom themes defined in ./themes

Start by inspecting the existing React and Material-UI theme setup, including the ./themes directory, then run the UI to locate where a visible control can be added. Done means users can switch between the default white theme, the dark theme, and any custom themes defined in ./themes.

Written by the indexing model from the issue text.