Convert SCSS variables to CSS variables and allow admin to modify them

[maisieccino]

Things like colours and fonts would be great to modify directly from the event admin page.

We could achieve this by converting all the existing SCSS variables to CSS variables, and then using the default SCSS variables as fallbacks. The advantage of this is that if the user does not have a browser that supports CSS variables, they will still see the default Omni Dash style.