dev.to apparently does not use Flarum, instead they're using Forem, not to confuse these two.
If you are looking for themes to use with Flarum, check out the Themes tag.
Having a truly unique look will most likely require you to create a custom extension or CSS. You can check already existing threads, or make your own CSS that you apply under "Appearance" in your administration area.
Here are a few communities that do use Flarum and custom CSS, for inspiration and taste of Flarum's great customizability. You could technically ask them to share a bit of Flarum customizability tips: