Vivaldi team has already added configuration panel for Reader Mode for version 1.11 which allows user to change font, adjust line height, column width and switch to dark or light theme while in Reader View. But to access the Reader Mode settings, you need to open the sidebar (tip: use F4 shortcut for that) and click on the panel icon above the Settings icon, which is not convenient.
That’s why Vivaldi team has ditched that panel and provided quick access to these by adding another set of controls on top of the page, which appears when you click on the cog wheel.
Vivaldi makes Alt+R as default Keyboard Shortcut for Reader Mode
The settings you apply for reader mode in Vivaldi browser -for instance, the dark mode or light mode you set -will be saved and won’t be cleared even when you close current session and reopen a new one. And you can reset Reader view settings.