[web] introduce AppThemeWrapper

Summary:
This diff introduces a new component that wraps the contents of the App component, and the job of this component is to update the data attribute based on what the activeTheme of the app is. The subsequent diff will introduce the css variable list that will use this data attribute to show dark or light mode color values

Sources I used to help me with this implementation:
https://css-tricks.com/easy-dark-mode-and-multiple-color-themes-in-react/
https://stackoverflow.com/questions/56300132/how-to-override-css-prefers-color-scheme-setting

Test Plan:
Please see the demo video below where the theme of the app changes based on the activeTheme redux state

Reviewers: atul, inka, rohan

Reviewed By: atul

Subscribers: ted, ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9580