Microsoft adopted the Chromium framework last year and since then the company has been adding features from the Classic Edge to make the new Chromium Edge feel more like Microsoft. Recently Microsoft added the password reveal option, reader mode and others to Chromium Edge. All these features were present in the old Edge and are now a part of the new web browser.
With the new update, Edge Canary users will be able to select from a large variety of colours and fonts for the Reader Mode. Along with that, Microsoft recently added Grammar Tools which should improve the reader mode altogether. Edge users will also get an option to change the background colour while using the Reader Mode which should be helpful for extended reading sessions.
The new features are available in Microsoft Edge Canary v80.0.351.0 or later. To use the feature, you will need to open a website and immersive reader icon to reveal the Text Preferences option which will give you access to the new features.