Updated Blog theme has layout errors

(re-writing this as my first attempt was rushed and unclear)

Since I last used it, the Blog theme appears to have been amended. The structure and CSS is different to that on another of my older Blot sites.

The changes appear to introduce some issue that are consistent across multiple browsers and machines - for example content in the right margin is now positioned off the page and pagination text is positioned top-left over the top of itself. I assume these are theme errors because they're visible on the theme demo site too and differ from the screenshots provided.

Has anyone identified the issue (although the HTML structure has changed, it looks like the CSS may be the issue here?) and / or is there a way to get the old theme to use, since the theme is unusable as-is.