How to add borders to tables?

The Flamingo skin (and some other skins) don't show vertical borders by default. If you want to change that you can add a  StyleSheet Extension (SSX) xobject (i.e. XWiki.StyleSheetExtension) to any wiki page of your choosing (you can create a new page just for this for example), with the following content:

.main th, .main td, .wiki-table th, .wiki-table td { 
    border: 1px solid #E8E8E8; 
    padding: 3px 4px; 
} 

You could also decide to have some background colors for table headers by adding:

th { 
    background-color: #F3F3F3; 
    font-weight: 700; 
}

And if you don't want the table to take the full width you could add:

table { 
  width: auto; 
}