I'm betting themes goes away, since Gmail is the only Google interface that's unevolved enough to let users actually pick the way they want their client to look.
Ugh, I need themes to differentiate different personal mailboxes or various work related inboxes. There are days I might have 5 or 6 google inboxes open at any given time.
In that case you can still use custom CSS / user styles.
I did this for a while to cope with some of Google+'s redesigns, and came to realize that you end up in a constant battle of dealing with your custom CSS being broken by updates and changes to the website, and that you are better off just suffering through what a given website wants you to use, if you're going to use it at all.