Is there a simple way, the icons of the currently used icon theme to use? Or must be adapted to the code?
I know I can create a separate folder under "pix". But I had to copy the icons.

I'm not sure I understand, if you are talking about creating an icon theme based on another without copying the files, it's not currently possible :(
I plan to eventually use the freedesktop icon standard, that allow this kind of things.

Currently the icons are searched in this order :
($theme being the name of the theme, and using the default install path as example)
~/.config/gmusicbrowser/icons/
~/.config/gmusicbrowser/icons/$theme/ if it exists or /usr/share/gmusicbrowser/pix/$theme/
/usr/share/gmusicbrowser/pix/

I mistakenly thought it was like below instead, not sure why I did as above.
~/.config/gmusicbrowser/icons/
~/.config/gmusicbrowser/icons/$theme/
/usr/share/gmusicbrowser/pix/$theme/
/usr/share/gmusicbrowser/pix/