Adding style with the Project Styles inspector
Structural Styles: Styles that apply to all items of a certain type. 
Named Styles: Custom styles you can apply wherever you like. 
Font Preview 
Font Controls: Choose a font, a color for the text, and a color for the background. 
Task Bar Color Controls: Set up colors for incomplete and complete tasks. Of course, these are only available for tasks, groups, and milestones. Structural Styles: Styling many items at once
You can change the style of a lot of items at once, based on what they are. For example, you could change the font size for the whole project, make all resource names italic, or make all completed tasks gray.
How to set up structural styles
In the list of Structural Styles, select the one you want to change.
Use the style controls on the right side of the inspector to change the default style for everything of the selected type.
Available Structural Styles
Whole Document — Everything throughout the entire document; it can be overriden by the more specific styles listed below it.
Column Titles — The headers above the outline views and charts, and the sideways resource names in the resource timeline.
Notes — Notes attached to tasks and resources.
Tasks — All tasks, but not groups or milestones. Note that the Completed Tasks and Overdue Tasks styles can override this.
Completed Tasks — Any task whose completion is at 100%. You could, for example, make tasks' Gantt bars turn gray when they are finished.
Overdue Tasks — Tasks which were scheduled to complete in the past, but are not complete. You might like to turn these bright red so that you can see where your project is getting behind.
Task Groups — All groups of tasks. By default, their titles are in bold text to distinguish them from single tasks.
Milestones — All milestones. By default they are styled the same as regular tasks.
Resources — All resources, how they appear in the resource outline and as assignments on the Gantt chart.
Precedence
Structural styles are the most general type of style, so named styles or ad-hoc styles can override them.
Named Styles: Creating your own reusable styles
You can make up your own styles and reuse them throughout the document.
How to set up named styles
Click the plus button at the bottom of the styles list to create a new named style.
Then, while you have the named style selected, use the controls on the right side of the inspector to change its attributes.
Once the style is set up how you want it, apply it to an item in the main window by either dragging it and dropping it on the item, or by selecting the item and then pressing the appropriate function key.
You can keep applying the named style to items, and any further changes you make to the named style will get applied to every item that has it applied.
Precedence
Named styles take precedence over structural styles; if a structural style and a named style are both applied to the same item, the named style wins.
Ad-hoc styling: Styling items individually
If you just want to apply a style to something without any special logic or updating, you can use an ad-hoc style.
How to apply an ad-hoc style
In the main window, select the items, or the part of a note, you want to style.
Use the right side of the Project Styles inspector to style the selected items.
Precedence
Ad-hoc styles have the highest precedence. If an item has an ad-hoc style applied to it, that style wins over any named styles or structural styles.