Am i the only one that sometimes witness spotify playing randomly two songs at the same time ( one that I can stop and second one i can't) What can I do to repair that error?

i want to create my customized theme and edit my Spotify's UI
but i have difficulties finding what i need.

The official documentation seems too simplistic in its introduction to the user.css file.

how can i create the user.css file when i dont know its structure?
where can i get to know Spotify's CSS structure?
By using chrome developer tool?
but how can i get it in my Spotify client?