Increase visibility of selected item background in dark themes

Closes #8419

app/src/main/java/eu/kanade/presentation/util/Modifier.kt

@@ -22,7 +22,7 @@ import kotlin.math.roundToInt
 
 fun Modifier.selectedBackground(isSelected: Boolean): Modifier = composed {
     if (isSelected) {
-        val alpha = if (isSystemInDarkTheme()) 0.08f else 0.22f
+        val alpha = if (isSystemInDarkTheme()) 0.16f else 0.22f
         background(MaterialTheme.colorScheme.secondary.copy(alpha = alpha))
     } else {
         this