The Theme Explorer contains every element you may use to create your theme. This is also the place where you can add or delete an element. Simply right click an element (e.g. Widgets). Once an element is created you can also right-click that element (e.g. Widget1) and rename it.
You can also use the search if you get lost when looking for an element you created.
IMPORTANT: You can select the top element in the theme explorer (MyTheme on the screenshot) and resize it using the properties pane (default: 1024x768). The editor also uses a inheritance for the fonts. The Font you select as the default font of your theme will be the default font in every single widget etc. - but you can still change fonts for a specific widget, you just have to think about that.