Make selected boxes light purple in colour

Currently, the selected boxes are light cyan in colour. This
does not match the purple header and footer. Hence, make the
selected boxes light purple in colour.
Centre noscript message

The noscript message that appears when JavaScript is disabled is
not vertically aligned at the centre. This is due to the active
practice panel elements that are empty but occupy space above
the noscript message. Disable all practice elements by default,
and modify the HTML layout so that the noscript message appears
to be centred.