You can change the style of text (font, size, text face [e.g., bold, italic, underline], and color) within idea windows via the Text menu. There are individual sub-menus for modifying each of these text properties.
In addition, Idea Keeper allows you to define a default text style (via the Default Settings Sub-menu nested inside the Edit menu) and up to 10 style templates. Each of these style templates stores the style of text (font, size, text face [e.g., bold, italic, underline], and color) for quick formatting. The default text style is automatically applied to newly created ideas.
See the idea or look in the "Style Templates" submenu in the Text menu for a description of keyboard shortcuts for these features.