Introduce a ThemeProvider and useTheme hook

[SjaakSchilperoort]

User story

As a developer,
I want the components library to provide a ThemeProvider,
So that our apps can apply custom themes while sharing consistent default styling.

Context

See #85 for context

Functional requirements

• Create ThemeContext in the component library
• Add a useTheme hook
• Wrap apps with ThemeProvider
• Migrate one shared component to useTheme