You can create as many themes as you like. 

If you cover news and sports, it makes sense to create separate themes for each. Additionally, you can set up different themes for different websites or domains and adjust them to match the look and feel of each site.

You do not need to create a separate theme for web/app versions or light/dark mode. 

Info on flipping live blogs (Sorting):

Since there is also the option to change the sorting of a liveblog from oldest to newest post after coverage has ended (which can be set in the theme), it is a good idea to create a separate theme for flipped live blogs.

If you edit a theme and change the "Sorting", all live blogs using this theme will be affected. Therefore, if you want to flip a live blog after coverage, it is better to change the theme ID instead of modifying the theme itself.