This patch refactors the config forms LESS to use defined variables for

colours. These colours were being used in Review Board's account pages

and this change will allow the colours to be imported (instead of the

current hardcoding that occurs).
The CSS for config forms lists has been slightly tweaked so that they

have the same top and bottom border. Previously, there was no bottom

border on the list -- it was just the border-bottom from the last

element in the list, which uses a different colour from the border-top

of the entire list.
Additionally, support has been added for a new

.config-forms-list-empty element that is intended to be a sibling of

the .config-forms-list. It will only be shown when the list has n

children. This is a generally useful pattern for supporting

placeholders that don't have to be dynamically added and removed from

the page. This is a refactor from a pattern I was using in Review

Board.

Tested with an upcoming change for Review Board.