Update the experimental struct color to have more contrast against de…

…claration highlight. (#32183)

src/VisualStudio/Core/Def/Implementation/Experimentation/EnhancedColorExperiment.cs

@@ -171,7 +171,7 @@ private sealed class EnhancedColorApplier
             private const uint DarkThemeLocalBlue = 0x00FEDC9Cu;
             private const uint DarkThemeMethodYellow = 0x00AADCDCu;
             private const uint DarkThemeControlKeywordPurple = 0x00DFA0D8u;
-            private const uint DarkThemeStructMint = 0x008CC77Eu;
+            private const uint DarkThemeStructMint = 0x0091C686u;
 
             private const uint LightThemePlainText = 0x00000000u;
             private const uint LightThemeIdentifier = LightThemePlainText;