I am not sure what the OP was requesting, but here's my take on trying to change the font size in the Vivaldi email client.
The Zoom bar affects only the reader pane (the one that the text of the currently selected email appears in). It does NOT affect the font size in the pane that shows the list of emails, nor in the folder listing panel, nor in the email drafting pane (when you're writing an email, the slider is grayed out). IMHO it should affect all those panes--if I need a bigger font in one, it seems likely I'll want a bigger font in all. (Note that this is about the screen size, not about changing the font size in the actual email I'm sending someone.)
The <CTRL>+ and <CTRL>- keys work similarly: they only affect the text in the reader pane. Again, I think they should affect all the panes.
The Zoom slider and the <Ctrl> keys are only effective for the current session. If I close and re-open the email client, or the entire browser app, my embiggened settings go away. There ought to be a way to retain my preferred font size across sessions, at least for the email client (unclear whether this should be done for ordinary http(s) sites, which already vary in font size). Perhaps this permanent change could be made in the Settings.
