For example. I found a simple online counter. Can I install it on the server with yarn and then insert it into the code without any problem to make it work?
סיכום מבוסס בינה מלאכותית
shaco wants to install an external application from GitHub, a simple online counter, on their Discourse server. However, riking explains that customization in Discourse is restricted to ensure stability, and built-in view counters are already available.
shaco is frustrated about the limitations, citing the abundance of ready-made solutions on GitHub. merefield responds by highlighting the importance of learning about Discourse theme customization, which allows for the reuse of existing CSS code. A beginner’s guide to using Discourse themes is recommended for getting started: https://meta.discourse.org/t/beginners-guide-to-using-discourse-themes/91966.
No, customization in Discourse is a little more restricted than that to keep everything working. Also, Discourse already comes with built-in view counters on topics.
Check out Customization > Theme for an idea of what you can create with the powerful theme customization system.
Okay, but the internet is full of unlimited possibilities. On github I have 150 million different ready-made solutions like buttons, css tooltips, counters etc. Why can’t I install it? It’s a pity that you can’t use so many ready-made solutions but you have to limit yourself to heavy plugins or write yourself if you still have such knowledge and time for it…
It’s a tough life, huh? 
It’s amazing there are so many well paid software engineers, given that on most platforms you can just chuck stuff together and it just works?
Ahem …
If you invest the time to learn how you can re-use loads of stuff, especially CSS.
Checkout Beginner's guide to using Discourse Themes to start off and keep going!