To style a menu use the CSS Injector module, see the FAQ section Working with Cascading Style Sheets (CSS).
The key is to target the correct selector. In the example above the suggestion was made to locate the Level 1 menu in the Top panel first. Using Firebug or another developer tool, locate the block surrounding this menu. You will see in the Firebug window that the id of the surrounding div is block-menu-block-1.
With this information, you can write a rule using the CSS injector which will format your menu, e.g.,
#block-menu-block-1 li {
display: inline;
list-style-type: none;
padding-right: 20px;
font-size: 1.5em;
font-style: bold;
padding-right: 2em;
padding-top: 1.5em}
For more information about styling menus see, Menu block (Drupal 7) at Drupal.org.