Dark Mode for web

[peterklingelhofer]

Description

This will be an iteration on #48. #48 provides an initial dark mode toggle for the electron app. However, since we're eventually aiming to integrate this locally in the stream-app and potentially, if all goes well, host this online, we will need this to be functional for web users as well, so it will work with yarn start.

Potential solution

There's a global state using useContext and useReducer, check it out in contexts/globalState.tsx.