Helen, upgrade process you followed was absolutely right to upgrade it perfectly, despite a huge gap in between. I haven't personally read through the upgrade documentation (or may be I had a decade back on my first upgrade), but I suspect a crucial thing missing from documentation is looking into plug-ins first and that is what I recommend people upgrading, to make a list of additional plug-ins installed and look into plug-ins directory to see if the plug-ins are updated to the moodle version they are updating, in most cases plug-in developers do upgrade plug-ins before hand or right on release of new moodle version. however moodle 3.9 is exception for still many plug-ins are getting there to be supported by 3.9. This brings to Adaptable which still isnt, so people switched to Moove theme (as of now, most downloaded theme) so assumption is people switching to that.
This, I recommend to you as well, to switch theme as of now, you can choose to see which theme suits you well by looking into most downloaded plug-ins (
https://moodle.org/plugins/stats.php) and you'll find some 5-6 themes listed there that were most downloaded. which ever suits you well and supports moodle 3.9 as well.
@Rick, Adaptable is a fork of BCU (Birmingham City University) theme, later developed to be used in Coventry University where Jez (Lead Developer) moved to from BCU, that is why I see Helen's affiliation to apply it on Coventry City Council