With css this is not possible, this is only for styling and i don't know any other way currently in joplin, except maybe the joplin-note-tabs plugin ...
I believe there's a plugin that allows you to store and rerun searches. Not sure what it's called, but it may be along the lines of "bookmarks" or "favorites" and the like.
It could be done using AutoHotkey if you're on Windows. You'd assign a macro to a key that would send inputs for Ctrl-Shift-F and then -tag:*. Not sure what equivalent tech exists on other operating systems.