You can use the Font properties in Microsoft Expression Blend to format the text in your application. If you use a RichTextBox, TextBlock, or FlowDocumentScrollViewer object, you can selectively apply formatting to particular words or characters in the object. With all other text object types, the formatting properties that you set will affect all text in the object.
You cannot animate the properties under Text in the Properties panel except for the FontSize property. If you animate the FontSize property, you must set the property for the whole object (in other words, you cannot animate the FontSize property when you are in text-editing mode).
Select the text object that contains the text that you want to format, by using either the Selection tool or the Direct Selection tool .
To format only part of the text in a TextBlock or RichTextBox object, right-click the object, click Edit Text, and then drag the pointer to select the text that you want to format. Otherwise, continue to the next step.
In the Properties panel, under Text, click Font, and then do one or more of the following:
Select a font from the FontFamily list to change the font for the text.
Select a size from the FontSize list to change the size of the text.
Click Bold to make the text bold.
Click Italic to italicize the text.
Click Underline to underline the text.