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You'll need to subclass NSScroller and draw everything yourself. Study the NSScroller documentation and play around a ton with overriding the various drawing methods. I did this a little while back for work, so can't post any code, but I did override drawRect: where I manually call drawArrow:highlight:, drawKnob, and drawKnobSlotInRect:highlight: which were all overridden.
 
I was able to do it! :)
Here is the code I used for anyone who wants it:
Code:
-(void)drawKnob{
	NSRect knobRect = [self rectForPart:NSScrollerKnob];
	NSRect newRect = NSMakeRect(knobRect.origin.x+1, knobRect.origin.y, knobRect.size.width - 2, knobRect.size.height);
	NSBezierPath *path = [NSBezierPath bezierPathWithRoundedRect:newRect xRadius:5 yRadius:5];
	NSGradient * gradient = [[NSGradient alloc] initWithColorsAndLocations:[NSColor colorWithDeviceRed:(float)173/255 green:(float)185/255 blue:(float)203/255 alpha:1.0],0.0, 
							 [NSColor colorWithDeviceRed:(float)117/255 green:(float)135/255 blue:(float)164/255 alpha:1.0], 1.0, nil];
	[gradient drawInBezierPath:path angle:0];	
	
}
 
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