A theme includes a background and a default font (font, color, and size). When you apply a theme to a blank page you will see the background be applied. Any text you then add will be in the theme’s default font. When you apply a theme to an existing page the background will be changed to theme’s background and all text on the page will be changed to the theme’s default font. If you undo applying a theme, the background will change back, the font and color will change back, but the size will not change. So be careful applying themes as you cannot successfully undo applying a theme.
The below video has step-by-step help on how to apply and create themes for your lesson creation.