I implemented referencing theme colors by name in user CSS. 

This allows one write user CSS like this while taking advantage hue shifting etc: 

.MPartStack {
	swt-unselected-tabs-color: #000 
'#InactivePartGradientStart' 
'#InactivePartGradientEnd' 
'#InactiveOutlineColor' 
'#InactiveOutlineColor' 
'#InactiveSelectedTabEndColor' 
0% 49% 55% 99% 100%; 
}

.MPartStack.active {
	swt-unselected-tabs-color: #000 
'#ActivePartGradientStart' 
'#ActivePartGradientEnd'
'#ActiveOutlineColor' 
'#ActiveOutlineColor' 
'#ActiveSelectedTabEndColor'
0% 49% 55% 99% 100%;
}