in bender, you will be altering the functions.php if you want changes on the main page content area. the main.php edits the sidebar on the home page. to edit the public end results page you will be editing the item.php page, if you want to edit the sellers page to submit an item, that would be on the item-post.php. all the files are in the osclass/theme/bender folder.
the default styles are what you get, if you want to alter any styles go to the main.css in the bender css folder.
I do not know where to alter languages, perhaps start n the benders language folder. and perhaps where you see this type of code <?php _e("Search by Location", 'bender') ; ?> you can change that "Search by Location" part to what ever you want.
I hope this helps you get started...