joomla is pretty much the same thing, but it is in active development, I think mambo was put on the back burner when they split ways
it started as mambo, then the owners parted ways and the one owner got control of mambo and the other retained rights to the script and called his joomla...joomla is very active, mambo is pretty stalled
586 templates to choose from -
http://templates.joomlaya.com/
as far as removing stuff, you will just have to get in there and play around, every template is different, and if I told you now how to remove it and you installed another template then you would have to find the info and remove it all over again