Modify wordpress theme to add custom functionality

Good evening. On my adult site I use the wp-script retrotube theme like many others.
What I'd like to do is to start adding fairly simple functions, for example putting a counter in the home page with the videos added today, adding a widget in the sidebar with custom functionalities... in short, small things, nothing too complex.
I'm proficient in java, python, and mysql. However I know little about php and javascript, but with the right resources I'm a quick learner. That's why I'd like to know if you can recommend any guides or resources on the subject, or point me towards the best practices to do.
Thank you.