I using the theme phpkaox, but after upgrading XOOPS Core, the menus displayed horizontally instead of vertically
The theme phpkaox is not part of Xoops core more.
In earlier versions of Xoops core, there was some theme definitions in xoopsroot/ xoops.css
They are removed, so you should include them in your themes CSS file
The missing lines are
#mainmenu a {text-align:left; display: block; margin: 0; padding: 4px;}
#mainmenu a.menuTop {padding-left: 3px;}
#mainmenu a.menuMain {padding-left: 3px;}
#mainmenu a.menuSub {padding-left: 9px;}
#usermenu a {text-align:left; display: block; margin: 0; padding: 4px;}
#usermenu a.menuTop {}
#usermenu a.highlight {color: #0000ff; background-color: #fcc;}
The linebreak in the menu is created by
display: block;
This Q&A was found on XOOPS Web Application System : https://xoops.org/modules/smartfaq/faq.php?faqid=698