Hi Chris,
As DSpace 7 introduces and entirely new User Interface, built on Angular.io, any old customizations to the DSpace 6 XMLUI or JSPUI will not be possible to directly move over into DSpace 7.
So, the simple answer here is that you'd have to re-customize the DSpace 7 user interface, as it's entirely different and there's no migration path from the old UIs.
That said, I will note that in customizing the DSpace 7 UI, it may be possible to perhaps "borrow" some styles/concepts from old UIs. In the DSpace 7 UI, we are still using
Bootstrap (now v4, but XMLUI/JSPUI used v3) and CSS/SASS (
SASS is also supported in XMLUI's Mirage2 theme). So, borrowing
individual styles might be possible, but again there will be no way to copy an entire theme (or stylesheet) from DSpace 6 into DSpace 7.
Tim