The Window menu lists all the possible windows in ATOMS - those which do not actually exist are greyed out. Select a window to make it active, or bring it to the front.
When a Text window or either the Powder or Precession graphics window is active, the first item in the menu is a Font selector. In the Text window, this acts on the entire text in the window and the font applies to all Text windows. The font for the powder and precession windows applies to both windows and to all text, except that the index lettering in the Powder window is 0.4 times the size of the lettering for legends.