Comment 3 for bug 1402236

While the feature is not implemented as requested yet, there is some progress.
Since 2.1, you can make a color schema work only on specific regions, like

<Scheme>
  <Name>foo</Name>
  <Filters>
    <HSVTweak>
      <HConst>90</HConst>
      <SConst>-30</SConst>
      <VConst>-30</VConst>
    </HSVTweak>
    <Add>
      <Amount>-40</Amount>
    </Add>
  </Filters>
    <Style src="skin:foo.qss"/>
</Scheme>

or load specific stylesheets

<Schemes>
   <Scheme>
     <Name>Style1</Name>
     <Filters/>
     <Style src="skin:style1.qss"/>
   </Scheme>
   <Scheme>
     <Name>Style2</Name>
     <Filters/>
     <Style src="skin:style2.qss"/>
   </Scheme>
</Schemes>

https://github.com/mixxxdj/mixxx/pull/1501