First, navigate to the page or post that you wish to format the text of, and select edit. Then, make sure you are in the Visual tab of the text editor, not Text.

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Once in the Visual tab, press the Toolbar Toggle button on the editor.

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The toolbar allows you to format the text size, color, and other special formatting options for content in your post or page. Simply highlight the text you wish to format and select the option on the toolbar.