How was the UI created / edit a UI widget or element?

[joshuacbngn]

Hi! I've really enjoyed this project, and really appreciated the UI that you gave for ease-of-use. However, as an aspiring developer myself, can anyone point me in the direction of how the UI was made? I'm looking into creating a similar type of UI for other projects, and I liked how it is presented in this one.
Also, if I can comment on the one we are using here, the 'Tune' slider gets pretty difficult to use sometimes when I try to set 12 or -12, sometimes it goes to 11 or 13 or 14. Things like this I want to know how to fix myself.
Thank you in advance!