We just upgraded from 1.5.21 to 2.1.2 and no matter what style I use I am having an issue with everything being left aligned with a bunch of blank space on the right under the New Posts and Post Thread Buttons.
Well, that's the thing, I think. When setting up the style on 1.5 several years ago, I think the Default style was the one that was edited instead of editing a child style. Now, the default style in 2.1 is the edited style from 1.5.
Did you create the XF1.5 style yourself? If not, contact the coder for help.
Either way, though, you need to re-import the default Xenforo 2.1.2 style and overwrite your old style. You can take a backup (export) the old custom style but it is probably not ever going to function properly in XF 2.x.