Google is testing some tweaked icons within Gmail on the desktop to closely mimic icons seen on the Android and iOS apps.
The change only affects icons for things like the sidebar folder navigation and the actions listed at the top of emails. The change replaces the older filled-in gray icons with slightly darker outlined versions, though the general look remains about the same and equally “Googly.”
This change in iconography doesn’t just match Gmail’s Android app, it also matches changes Google has rolled out to Android and other apps in the last few years. These outline-style icons are the future if you’re Google, and they’ll probably eventually replace any filled-in icons across all the company’s apps and services.