If you are using Theme inside your application with reference to res/val/style and it's variant styles-v11 or styles-v14.Then you can surely achieve customize fastscroll indexer.
Append these attributes in your styles-v11 or styles-v14 inside your parent Theme attribute.
<style name="Theme.Zname" parent="@style/Theme.Sherlock.Light">
<item name="android:fastScrollPreviewBackgroundLeft">@drawable/_ics_fastscroll_label_left</item>
<item name="android:fastScrollPreviewBackgroundRight">@drawable/_ics_fastscroll_label_right</item>
</style>
Here as it is using ActionBarSherlock, so styles fastScrollPreviewBackgroundLeft and fastScrollPreviewBackgroundRight are appended in it works with API >11.
Put these inside drawable/_ics_fastscroll_label_left
