Just a punt, as I am not a theme developer, would you not add in something like:
ul.polaroids { list-style-type: none; margin: 0; padding: 0; background-color:#ffbf80; }
somewhere at the start of that code?
Seems to me that the list is being a traditional list and aligning each element to the left, whereas what you want in a block.
Alternatively, as this is for the front page, have you considered a table?
