To open the document properties panel, go to File > Document properties and the panel will open to the right of the editing area.
Alternatively, simply clear your selection by clicking outside the document and it'll show by default.
The document properties panel allows you to control various aspects of your whole document, such as:
Brand - change the theme colors and fonts of your document to match your brand (or switch back to the colors and fonts of the original template design).
Brand Colors - change the theme colors across your whole document.
Color Palettes - select from a pre-defined range of colors to apply to your document (note you won't see this option if you have a Brand selected)
Text Styles - change any of the fonts across your whole document (note you won't see this option if you have a Brand selected)
Spell Checker - change the dictionary language of the spell checker.