If you are using a theme to display the buttons, you *have* to modify
the theme to change the buttons in any way - wether the change is
button size, colour, or font. Changing the button size or colour
means using a image editor and replacing the buttons in the theme.
If you are not using a theme, then an image editor is needed to modify
the buttons, if they are graphics.
If you are using a text style for the buttons, then place the link bar
in a container (<span>, <p>, <div>, <td> - the exact container depends
on your page coding) and apply formatting styles to the container.
The Nav bar will pick up that formatting.