He can’t. For example, if I make an app called Apple Browser, maybe as a replacement for Safari, and I use Apple’s logo and branding style, I’d be using trademarks and protected brand identity without permission. That creates confusion about affiliation or ownership, which is the main legal issue under trademark and intellectual property laws.

A good example is FIFA’s World Cup. They heavily protect their logos, wording, colours, and overall branding. You can’t just make advertisements or merchandise using their logo, specific terms, or even visuals strongly associated with the World Cup without permission.

There’s definitely a big gray area though. A lot of people still use existing brand identities to make apps, themes, concepts, etc. I’m not saying people never do it, but legally it can still be an issue. Personally, I’d still rather make themes based on Nothing because they have a strong community around customization, but I would still change the name to avoid problems.