When developing themes when changing the device I'm using with developer tools, the store automatically falls back to the mobile theme, which is the default theme!

I have been redeveloping the front end and discovered some users are still on really old versions of safari that don't support import maps (So I can use versioned resources). I'd like to be able to set there theme to a fallback without having the "previewing theme" and the above also happening. Also I'd to be able to take advantage of some new CSS features. Our time to first paint and page speed is important to us.

Edit: Maybe even allow a url param that differ's slightly from sessionthemeid.