Implemented referencing theme colors by name in user CSS. This allows
one write user CSS like this: 

.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%;
}
Merge pull request #101 from Overruler/master

I implemented referencing theme colors by name in user CSS.