BuddyPress.org

I dont found the File for changing Buttons and textinput field etc.
I use the nikkon premium theme and want to use a dark buddypress Homepage Design. But i cant Chance Buttons lines etc.
This is all white or grey and I dont find where i need to change the colour of all white things.
I should do this in nikkon theme or in buddypress or i wordpress?
And in wich Files i can find the rigth css

I hope someone can realy help me.

Best
Max