Tip No. 1. - Don’t make UI design courses for developers if you don’t have a solid experience in it.
This reads like a harsh criticism; arguments would be welcome as to why you feel this way
Serif fonts are fashionable but a questionable choice for legibility. (HN is much more legible.)
The header image is unnecessarily huge.
There's a login box, but you won't spot it unless you're paying attention. Why would you want to log in anyway?
Overall, the design looks knocked-together in a hurry with no real understanding of either good layout or UX.
The app itself has poor layout spacing and - ironically - the colour choices and combinations are unusual and not what you would expect from an experienced UI designer.
To be clear, this isn't being picky for the sake of it. I would honestly be surprised if a majority of experienced UI professionals were enthusiastic about the current design of either the site or the app.