First, there’s no copyright on a general style, or theme. Nor can it be patented or trademarked.
So, legally, this is already nothing.
Second, this is not something Google invented – Google started with webmail, but many other companies had designed stuff like the in-frame editor, or other concepts before Google.
So, even if ProtonMail outright copied Google, Google, because they copied together many other people’s ideas, wouldn’t have reached the Threshold of Originality.
They're called Design Patents. They exist. Apple vs Samsung was quite a famous case over them with hundreds of millions in damages on the line. I have no idea if Google is using Design Patents on their products but can if they choose.
Your first sentence was broader than that. Far as EU, they're called registered designs. I don't know much about them past that they exist. They're also in Hague Agreement of WIPO which Switzerland is a party to. Apple's attack on Samsung in Germany was about them copying the interface and style. Situation isn't as clear cut to me given the above.
Even if nothing applied in Switzerland, the countries supporting such things might block their local presence there under grounds of violations. This risk actually is why some companies, esp Chinese, won't do business in the U.S.. If this played out, they'd be blocked from serving customers in U.S. or any nation that upholded the claim against them.
So, legally, this is already nothing.
Second, this is not something Google invented – Google started with webmail, but many other companies had designed stuff like the in-frame editor, or other concepts before Google.
So, even if ProtonMail outright copied Google, Google, because they copied together many other people’s ideas, wouldn’t have reached the Threshold of Originality.