Best way to edit style of others' Excel sheets?

So I recently started using Excel, and for me having dark mode for everything I consistently use is a must. I know that on Windows, there's a simple dark mode toggle button, but that doesn't appear to exist in the Mac version.

I currently have a custom default template that I made by clicking 'Format Cells' and then changing the background and gridline colors to my liking.

But if someone sent me a sheet where most cells had a white background, I would at least want to turn the brightness down or something. Like in this example, things already look so much better when it gets kind of dim after selecting all cells.

What's the best way to get a darker effect that I'm looking for? I heard about VBA for scripting purposes, so I could potentially try to learn how to write a script that can automate darkening files.