Added css variables with four defaults to speed retheming

Adds CSS Variables to allow the login to match the inside theme: supporting Finsbury Park, Aah and Shoreditch, plus another style.

Also makes a couple of small html tweaks, removing the #bootstrap-theme container as it's not used, using flexbox to centre parallel elements.

Original changes with screengrabs here: https://lab.civicrm.org/nicol/standalone-ui