Change the overall appearance of a notebook by editing the styles in its stylesheet.

Open a notebooks stylesheet for editing

You can customize individual styles in a notebooks stylesheet.

To change a style in the current notebook, choose Format Edit Stylesheet:

Choose a style to modify

In the stylesheet that opens, choose the style you want to modify:

Adjust the style

With the style cell selected, use the menus to adjust its stylefor example, changing the color of the text:

All cells with that style in the current notebook change accordingly:

Notes
If the current selection is in one of the commonly used styles, you can determine the name of the style by looking for a check mark in the Format Style menu: