WikimediaUI theme: Variablize PopupWidget values
WikimediaUI theme: Variablize PopupWidget values

Variablizing PopupWidget CSS values.
Also
* re-aligning close button,
* relying on same implicit height logic like elsewhere instead of being
prone to scalability issues with explicit way and
* cleaning-up selectors.

Change-Id: Ie2a2786d06505b2fa9e8b7ddfd85ddfab99b1af9

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui