The website you linked to appears to be using the Bootstrap framework in their theme design (based on what I saw from their source code). You can learn more about Bootstrap from their official website and use it to create a similar/identical theme or you could pick and choose whatever specific feature you like.
If you are only interested in implementing the same post presentation method (where clicking on the title opens the post and another click collapses it) this is called an accordion in website design terminology.
To achieve this using the Bootstrap framework, refer to the official Bootstrap documentation on using the collapse behavior to create an accordion: https://getbootstrap.com/docs/4.1/components/collapse/#accordion-example
After you create your accordion, you then just tie it with your WordPress loop in order to place your content in the right place.
However, if you don't want to code a new theme using Bootstrap or create this from scratch, then other options are to:
- Use a WordPress theme that is based on the Bootstrap framework, or
- Use a theme that implements an accordion (without Bootstrap), or
- Use a WordPress plugin that allows you to present your posts in an accordion format regardless of what theme you are using.
You can find these type of themes and plugins on the WordPress.org website and there are many options to choose from. I know they are available but I haven't personally used any of them recently which is why I have not linked directly to specific themes or plugins.
All the methods and options I mentioned above have their pros and cons and much depends on how much time you have to work on this as well as your current skills in coding, among other things.
I hope you find this information useful and please do not hesitate to let me know if you need more help or clarification.