User specific style sheet selection added

1. Light (main.css) and dark (main_dark.css) style-sheets added. The user can select preferred style sheet within Admin → Account
2. Users action method (feature) added to DapaExplorer class. This feature allows a DataExplorer user to see if any other user has currently selected a specific canvas-element. The other user's first name is shown at the left-hand top corner of the canvas-element.