tableView侧栏索引的背景色默认是白色，但是可以通过以下属性修改
1.修改index文字颜色
@property (nonatomic, strong, nullable) UIColor *sectionIndexColor NS_AVAILABLE_IOS(6_0) UI_APPEARANCE_SELECTOR; // color used for text of the section index
2.修改背景色
@property (nonatomic, strong, nullable) UIColor *sectionIndexBackgroundColor NS_AVAILABLE_IOS(7_0) UI_APPEARANCE_SELECTOR; // the background color of the section index while not being touched
3.修改点击后的背景色
@property (nonatomic, strong, nullable) UIColor *sectionIndexTrackingBackgroundColor NS_AVAILABLE_IOS(6_0) UI_APPEARANCE_SELECTOR; // the background color of the section index while it is being touched
例如：
//将背景色改为透明
_historyPropertyTableView.sectionIndexBackgroundColor = [UIColor clearColor];
