This extension is no longer being developed. Keeping it up to date with Gmails DOM changes has proven too time consuming. The Simplify Gmail Extension which has over 95K users is recommended as a substitute.
Inbox Theme for Gmail
Google is shutting down Google Inbox next year, asking users to switch over to Gmail. Although many Inbox features have made their way to Gmail, the visual experience is still quite different.
This Google Chrome extension brings Gmail closer to Inbox's visual style. While Google is likely to update Gmail's design to follow their Material Design guidelines in the future, this extension serves as a stopgap solutions for us who are used to Inbox.
How it works
Just install the extension on the Chrome Web Store, then simply open Gmail. To go back to regular Gmail, just turn off the extension.