Utilise theme to provide app-wide styles (colours, text styles etc.)

Currently, only the red and green colours used for notifications are provided through the theme object exported by the [apydoxv1 theme](https://github.com/freshwebio/apydox/blob/master/portal/src/styles/themes/apydoxv1.ts). All theme colours should be provided by this theme object.

This will allow for switching between multiple themes seamlessly and provide better structure and a single source of truth that allows for more control and visibility in the case of design changes.