underbone,

ladoga explained it to you but maybe you haven't understood it yet.

The file /usr/share/themes/base/meegotouch/libsysuid-screenlock-nokia/style/libsysuid-screenlock-nokia.css needs to be opened to be edited right? How would you open it on terminal? You need to open it in an editor to do so.

Therefore you need to tell terminal to open this file in which editor (nano, vi etc).

tl;dr type the following in terminal:
Now edit it and when done press Ctrl+X to save.