Enable "Download Messages for Offline Viewing and Searching" to access your mail without internet connection.
Press F2 / ⌘ E to open Quick Commands.
Enable Desktop site from the Vivaldi menu on Android, to view desktop versions of websites.
Click on the "moon" button to switch Vivaldi Webmail to dark mode.
Install the Linux ARM version of Vivaldi on Raspberry Pi.
Click on the padlock on the Address Bar to access Site Settings.
Hold down the right mouse button and scroll to cycle through tabs.