Notice also that a dotted line is around the check-box of Bldg2. You can add, change or delete features while the dotted line is showing. You should only have editing turned on while you are changing a theme, since you could inadvertently change something you didn't want changed. You will next add some features to the theme, then turn the editing off. To digitize a feature, you must use the appropriate drawing tool.
Several tools are available for drawing points, lines or polygons.
Slide down until the polygon tool is highlighted, then release. The toolbar icon should change to this polygon tool. Move the mouse cursor to one corner of a building and click once. Move along the edge of the building to the next corner and click again. When you get to the last corner before the one you started on, double-click the mouse.