The latest version improves compatibility with latest Hugo and adds several enhancements. This enables us to drop most of our vendored copies of the theme files.

Mermaid is now a theme option, so we don't need to overlay a copy of the partial with it removed.
Custom CSS has a theme option to load it, so we don't need to add it to custom-header.html.
It now uses the recommended colour style variant customization instead of copying the theme's CSS. Most of the sidebar seemed unchanged from the default, but I copied them anyway to prevent them changing.
Dropping the copy of the theme CSS means moving our customized fixed-max-width style on the body to the custom CSS.
I moved the footer into custom-footer.html so that most partials would not need to be copied; this does mean it's no longer a bubble that's the width of the content, but I always found that weird anyway. Some new CSS is necessary to make it centred properly.
I found many of the partials and the JavaScript were functionally the same as the original theme's copy, so I removed them.

Opened up a few pages and saw things were roughly equivalent, except the footer as noted above.
The only regression I believe is that the ToC is no longer automatically added on 'long' pages. However, it's still there on hover of the ToC icon.