First Vivaldi 1.11 snapshot for Windows, Mac and Linux platforms has been released and available for download. The snapshot apart from fixing bugs found in Vivaldi stable 1.10 has added configuration panel for Reader Mode aka Reader View.
When you enter reader mode for current page by clicking on icon in the address bar, configuration panel for it will be activated and visible if you’ve the sidebar open, which allows to change font type, font size, column width, line height and style of the page to light, dark or theme mode. If you want to revert to Vivaldi default Reader Mode settings for that page after modifications, you can by clicking on Reset button.