While searching for a simple map, I came across MapTiler.

Maps for developers
Mapping platform designed for developers. Publish interactive maps in your web applications and on mobile devices.
www.maptiler.com

Specifically, the following Basic Light looked good,

Maps
Free detailed maps of the entire world for your applications. Map hosting for your websites and products.
www.maptiler.com

as did the following Basic Dark.

Maps
Free detailed maps of the entire world for your applications. Map hosting for your websites and products.
www.maptiler.com

However, when I tried to use the above maps from the My Cloud page, I had difficulty finding them.

In the end, I was able to reach them from the following URLs respectively.

ThemeURL
Basic Lighthttps://cloud.maptiler.com/maps/positron/
Basic Darkhttps://cloud.maptiler.com/maps/darkmatter/

I hope this serves as a useful reference for anyone encountering the same issue.