Thank you for the reply. I forgot to come back here and post the solution to my issue, which was missing files from theme (don't know why).
Found an old thread that recommended doing a forced cpanel update, i.e. from commandline:
That did resolve that issue.
Again, thank you for the reply! My apologies for the mischaracterization of the issue — basically, the new login look for WHM and cPanel went away, and the old x style was there. I saw in... some log somewhere that I don't remember now that it noted the files were missing so falling back to that.... But I don't remember when I saw that vs. when I made this thread as I'm fighting several problems on the server.
But at least the solution that worked is now in this thread for anyone else having that problem!