Code: sed -i 's/margin-top: 6mm/margin-top: 0.99mm/' /usr/share/themes/base/meegotouch/libsysuid-screenlock-nokia/style/libsysuid-screenlock-nokia.css 6mm is original. Why 0.99, because you are able to change it back same command by changing the figures in the other way.
sed -i 's/margin-top: 6mm/margin-top: 0.99mm/' /usr/share/themes/base/meegotouch/libsysuid-screenlock-nokia/style/libsysuid-screenlock-nokia.css