How to design a Gtk3 application

I'm a student and I'm working on a project right now that needs to be done in C. We have decided to use Gtk3 and Glade for our application. This application consists in creating algorithm schemas with integrated animations for each object. For example insertion in a binary tree. We were also going to manage the cases for electronics and logic gates but I think we won't have the time.

If I'm talking about all this, it's because at the moment I'm busy implementing the parameters and more particularly the themes. I realized that with a css file we could easily change the colors for example. Except that I realized that no color combination was suitable for the application. This led me to think that it lacked design and maybe even ergonomics.I was all the more convinced when I saw an application in the same style as ours but so much more beautiful (Birdfont).

I have no idea how to make it more beautiful, especially since there are very few video examples of beautiful application development with Gtk. It's a bit of a silly question, I don't even know if it's appropriate to ask it on StackOverflow, but do you have any advice to give me, resources to help me etc.? I've done a lot of research and I must be doing it wrong because I feel like I'm going in circles.

I hope I haven't forgotten anything and made myself clear. Thank you in advance for your help.

I am attaching pictures of my current application.