There are two modules currently available in Conkeror for tabs, tab-bar.js, and new-tabs.js. There are plans to merge them into a single module in the future.
The first version is tab-bar.js, which is the older variant using a classical tab look.
The newer version is new-tabs.js, which is largely based on tab-bar.js, but takes a rather different approach to the actual tabs. Using CSS, it can be skinned by users according to their own tastes. A default skin is provided.
Absolute visual customizability through standard CSS. More info can be found at StylingChrome.
Mouse buttons for selecting and closing tabs can be customized (defaults to left and right mouse buttons, respectively). For more information, C-h v tab_bar_button_select and C-h v tab_bar_button_close.