Version 6: basic dark mode, new stream (Hackney Brook), new variables, fixes

NEW VARIABLES. Adds basic dark-mode support. Adds new stream: Hackney Brook, based on Finsbury Park (~90% port). Adds new CSS variables to all streams to support Finsbury Park's two main differences to Shoreditch and Greenwich: button icon styling (unique colours, background, border), and contact dashboard side tabs with active/hover border, similar to the 5.72 SearchKit UI, as well as some extra useful variables (e.g. dialog header border, notification border radius, etc).