Hi Osman,
This might be the best place to start: Theme Developer Quick Reference Guide.
There’s also a great deal of info on the pluginAPI here: Beginner's guide to developing Discourse Themes, including code demos.
You’ll also want to look at our CSP documentation if you’re adding scripts, so that they’ll be allowed to run: Mitigate XSS Attacks with Content Security Policy.
I hope that helps, but as always, feel free to send through additional questions.
Mark