Customize the phplist

I want to customize the phplist, such as changing its program background color but I do not even know where to find it. I’m not sure how the software folders are arranged and what each one is for. I’m using version 3.3.1 in ubuntu server 16.04 Can someone help me ?

look under /lists/styles make a backup first, then tweak away.

Thanks for the feedback and sorry for not responding sooner.
However I found what I was looking for in admin / ui