fix highlighting of chapter (#16418) #16432
                
     Merged
            
            
          
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fixes #16418
When selecting a chapter (for example go to https://lichess.org/study/q22lMoJU/94uYDPz7 and look in the chapters menu, to the left) it's hard to see what the active one is. This was both hard to see on light theme and on picture backgrounds. The simple css change made it easier to see what the active chapter was.