There are many core Windows 10 app update each day, but not all have obvious changelogs. In this case though there are changes that are front and centre in a big new version of Photos for all devices in the Insiders 'Fast' ring, across PC, tablet and phone. And, in due course, for everyone else using Windows 10 Mobile, regardless of production or Insider status.
Here's the summarised changelog for the new version, v16.1003.10012.0:
New option to choose either a light theme, a dark theme or just follow your system theme choice
Font changes in album titles, no more WP7-ish bold fonts
Photos are now animated back to thumbnail size after viewing, mirroring the 'expanding' animation when originally selected
Better album layout and smaller thumbnail sizing (three across on the editorial Lumia 950 and 950 XL)
Note the new theme selection (oddly called 'mode'!), plus the new, cleaner album titling and layout...
You can update Photos in the Store in the usual way if you're on the 'Fast' Insiders ring.