Add Themes for styling

[Haimchen]

1. User can choose a theme from the settings page
2. Theme is stored in local storage

When the app launches, the theme is retrieved from storage and provided to all components through Context: https://reactjs.org/docs/context.html

The Provider could

1. Provide the theme name
2. provide an Object that contains all variables belonging to the theme: { colors: { font: #237464, bg: #ffffff}}
3. provide whole style object with theme already applied