i cant select Module.css file from where you mentioned , but through FTP and selecting the /DesktopModules/WillStrohl.ContentSlider/Module.css i can.
But i already have the following css snipet :
background:url(images/loading.gif) no-repeat 50% 50%;
it still keeps it on the left side .
i tried removing the above and replacing with the one you provided till it doesnt make it go in the center