Replace custom theming with built in Emotion theming

[WVAviator]

https://emotion.sh/docs/typescript#define-a-theme

Define the theme type and use Emotion's Theme provider and useTheme hook.

What about a default theme?

[WVAviator]

Based on this thread: emotion-js/emotion#811
The best way to introduce a default theme if the user does not include a ThemeProvider is to import the default theme and use it if there is no ThemeContext provided. Not sure I like this - but maybe I can include it in a custom useTheme hook. I could also consider using styled components so as to receive the default theme as a prop with a default argument.