To open the document properties panel, go to File > Document properties and it'll open on the panel.
Alternatively, simply clear your selection and it'll show by default.
The document properties panel allows you to control various aspects of your whole document, such as:
Document Theme - change the theme colours and fonts of your document to match your brand.
Theme Colors - change the theme colors across your whole document.
Color Palettes - select from a predefined range of colors to apply to your document.
Text Styles - change any of the fonts across your whole document
Spell Checker - change the dictionary language of the spell checker.