I'd like to know it. I don't think that it can be substantially better. You have to compromise between the amount of customization you are allowed to do and the amount of options you have to pick values for.

I can't vouch for GPUI, I don't know how good its compromise is, but it bears enough similarities to other modern UI-toolkits. I don't think there is a better way, but if you know one, I'd like to know about it.

And, well, don't forget, it is a front-page example, that is not necessarily an example how one should compose their code, it demonstrates what you can do with GPUI. In real life code you'd probably move those children's construction into separate named functions, maybe you'd use some predefined styles to apply to individual elements, instead of picking fg/bg/text-colors, maybe sizes and some other attributes for each one explicitly, and maybe some other tricks.